Many people, even fairly sophisticated musicians, fail to realize how entirely different the organ and piano are as instruments. (But they both have keys!) Physically and musically the instruments need to be approached entirely differently. Seating an excellent pianist at an organ console unprepared is going to result in a lot of clunky, awkward playing—if they can figure out how to turn it on. (Same for trained organists sitting at the piano—although I know some who think they're doing just fine! Spoken as a fairly accomplished pianist who has been working seriously at the organ for 25+ years—and is still working at it!) There is also the matter of registration which non-organists typically don't realize is an issue. There's a lot to know! And this is to not even mention the pedals. (But, honestly, you could spend an entire lifetime playing appropriate music at a Catholic church and never need to touch the pedals. It would however be desirable to be able to use them!).

Judging from the question, I think princehal suspects this. So, my choice would be choice 4., at a minimum! Preferably, if the organ is to be played at church, I would recommend hiring a trained organist. But if your pianist is willing to invest a little time, see if you can get her to someone local for some introductory lessons, at least. In my area there are several people who will arrange to meet you at the organ you will be playing services on, which is a tremendous advantage.

However, the description of the instrument is not encouraging. If it's a nice piano, perhaps you are better off staying with the piano (spoken as one who is generally against using a piano). Any chance of some long-term planning to get a suitable instrument in there? And an appropriate budget for hiring musicians?

So MarkS, if I'm hearing you right, you'd say A piano is more suitable for liturgical accompaniment than an organ that is played without formal training. Is that accurate?

The organ is the best instrument for liturgical accompaniment, and I am generally against using the piano, particularly for congregational singing. No, what I meant is that piano might be preferable to the particular electronic 'instrument' at your disposal—but that's another thread! (Pipe organ purists—my side—vs. those that think digital/electronic instruments are a legitimate option.) If you are happy with it, then use it.

Best of luck!

Correctly has it been pointed out that the piano and organ are not at all alike, save only in that they share a keyboard by which are sounded, by fundamentally different means, diatonically tuned strings or pipes. Beyond that, the similarities diminish - particularly in touch, attack, articulation, manipulation of tone duration, expressivity, and on and on, not to mention the matter of how to handle and register the different ranks of pipes: which ones compliment each other, and which ones are not-so-good or even amusing combinations. Learning to discern what sounds right and what doesn't is an art in itself.

However, an intelligent pianist-musician may very well quite readily learn to feel the difference and adapt how to achieve a musical result on the organ. Learning or intuiting finger substitution for expressive linear execution is, perhaps, most fundamental. It won't be perfect or well-tutored, but it may, with some careful experimentation, result in a passable performance, one which would be preferable to the piano in church services. The organ is always preferable. (I did say 'organ', and not 'simulacrum'.) The piano is never a desirable instrument for liturgy. (I did say 'never'.)

At least as critical as touch, I think, is the matter of registering complimentary stops. Understanding the basic difference betwixt principals and flutes, the uniqueness of reeds, and a grasp of what mutations and mixtures are for will give the amateur a reasonable basis for decent playing. This knowledge may be had with a few lessons from a reputable organist, or, if necessary, from a good method book, such as Gleason.

A pianist who is being asked to do the work of an organist should ask him- or herself whether he is willing to engage in a minimum of study (with a teacher, or, if absolutely necessary, alone with a method book) which will result in at least passable organ playing. Some pianists have done quite well at this - relatively speaking. Others should not have bothered. This really hinges on the native musicianship and intelligence of the respective pianist.

In addition to the above one should wish to convey an ancillary-but-indispensable aspect of learning the organ: hearing it played well and assimilating what good organ playing sounds like. Whether one studies with a decent teacher or, of necessity, teaches oneself from a method, no real accomplishment can be made unless one knows and understands what good organ playing sounds like and brings his musical intelligence into play to imitate it. A musical imagination fired by some grasp of the effects of good organ playing is essential.

With such apprehension a talented pianist-musician can, with his or her native musical intelligence intuit much, and adapt his technique in such manner as to produce the sounds, the tonal ethos, of the organ with which he or she has become familiar.

There is no substitute for this aspect of learning the organ.

Attend recitals. Listen to CDs. Listen to literally anything you can imagine on youtube. Attend well-played liturgies at Episcopal cathedrals and such. If one is fortunate, he may find a Catholic parish with intelligent liturgy and organ playing - but this will be a relative rarity.

I'd been suspecting that the strong resistance against a pianist playing organ assumed that the pianist was musically oblivious...

Actually, not in my case. I was speaking from experience! I'm a former hot shot young pianist (Pre-College Juilliard, Tanglewood Piano Institute, six years at Boston University School of Music, etc..., he said, modestly) and I'm a pretty damn good musician. But it took a long time for me to make the adjustment. Perhaps my standards are higher than otherwise?

If we are talking about making it through a hymn on the manuals in reasonable shape, I guess that probably wouldn't take too long—with a little guidance, self-administered or not. I guess I wasn't really thinking in those terms.

Adam Wood for the win again.

Never tired of it.

Prince - your royal highness:
Whence the 'princehal' moniker?
It reminds me of my absolute favourite Sunday comic strip of years ago, namely, Prince Valiant, known also as Prince Hal. who flourished in those times approximate to the Roman empire's dissolution.
The art was unique in the comic strip world, quite tastefully beautiful and, for the most part, historically accurate.

As for autodictats - time was when they were a relatively rare species of self tutored geniuses who garnered great admiration, renown, and respect.
I just recently heard one, a young French pianist who has placed third in the Tchaikovsky Piano Competitions in Moscow.
Now, it seems, thanks to the internet, they are becoming 'a dime a dozen'.
It is good for education that the internet has opened up new worlds to many who, otherwise, would languish in ignorance.
However, there are invaluable aspects of intellectual rigour, professorial lineage, and other qualities to a systematic academic education that cannot be 'picked up' on the internet, or anywhere else.

Prince Hal is just Shakespeare role I like.

I played the Dauphin in a local production of Henry V. I suppose this makes us mortal enemies now.

Although in fairness, the English monarch shows great taste in Gregorian chant on the battlefield ("Let there be sung Non Nobis and Te Deum . . .")

Our former pastor used to compare maintaining a pipe organ to keeping up a Rolls-Royce. I thought that a bit of exaggeration, but pipe organ maintenance isn't cheap.

It's not cheap,...

Not cheap, but it pales in light of the cost of replacing
a simulacrum every 25 years (if, indeed, it lasts that long).
I find it disingenuous that those who rail about the ancillary costs of owning an organ
never mention those of owning a simulacrum (things will go wrong with it!) - not to mention its total replacement somewhere within three decades.
And that's not at all to consider what it doesn't sound like.

(And, I think that the Rolls-Royce comparison is a little gratuitous much! Another insouciant dictum tossed into the fray by otherwise unqualified [or unrepentantly biased] commentators.)

It's the initial sticker shock for the uninitiated, and for the initiated, it's the shock of having to explain that sticker shock to the uninitiated. Two circumstances emerge when discussing the price of an organ:

1. The pastor does not understand the product, therefore he cannot justify the price. Combine that with maintenance and other associated costs, and it becomes overwhelming for him to think about.

2. The pastor understands very well what he is purchasing, and the associated costs, but he cannot justify it to the parish finance council, because he knows THEY don't understand.

It's another case of serving money. A piano is immediately cheaper, an electronic is immediately cheaper, and so they just purchase those instruments instead of a proper pipe organ. They're not thinking about the longevity a pipe organ would have and that the instrument could easily serve the parish for at least 100 years with proper maintenance; they're thinking only about the here and now and what will make them look good to their "constituents." Spending $500,000+ on a pipe organ doesn't make sense when you could spend $50,000 on a grand piano, and use the remainder for things like wreckovations.

N.B.: The amounts listed above are informed estimates from prices listed by vendors, obtained through a Google search.

How many things in this world are maintenance-free?
Air conditioning systems?
Automobiles?
Organ simulacra?
Houses?
Plumbing?
Roofs?
Churches?
Yards?
Kitchen stuff?
Computer stuff?
Copiers?
Our own physical selves?
Anything?

None of the above and nothing else!

So why the apoplexy over organ maintenance?
I suspect that it's a handy non-reason for someone who doesn't want one in the first place and for whom any excuse will do. (And, it's a favourite of the simulacrum salesman.)

when you could spend $50,000 on a grand piano,

Or $2500. Or nothing.