There are a couple matters here:
1) Guitars and polyphony. There are some 16th-c mass sections intabulated for lute. If roomie wants to do that sort of thing, he should examine them...or even better, tune his G string down to F# and play them from tablature. You won't get everything into an intabulation; the lead lines are about it. And I suspect that such intabulations were never used for choral support; you wouldn't be able to hear them. When lutes started playing with choirs, it was late enough that they were probably playing some kind of proto-continuo.

2.) your main question: what to sing. I'm going to talk about editions first. They should be absolutely clear and legible, not cluttered with added editorial nonsense (dynamics and such). I suggest you settle on a beat value and stick with it (half note is most generally used), and avoid barless/Mensustriche editions until the group is well along. Their music reading experience has probably been with common-practice music, and if you're looking for good results at first, you shouldn't deviate too strongly from that. A beginning group shouldn't be expected to transpose music; notes should correspond to sounding pitch, and if it's necessary to do the piece at a different pitch, be prepared to enter it one way or another into a notation program and print it out down M2, m3 or P4 or whatever. Making music is hard enough without the notation getting in the way. Last night I sang a Requiem on about a half hour rehearsal, one to a part, and the Ordinary was in breve beat without barlines. It was very simple music, but the notation made it non-simple.

Repertoire: Most people start with music which is predominantly homophonic. There are good reasons for that, but it's also a crutch, and there's not so much of such music. I don't think it's necessary to have a texture be "busy". I do think it's advantageous to have music where most voices are in most of the time; most mistakes are made at phrase entrances.

3. Why "may be conductorless"? Can't you beat time and sing at the same time (yes, it's a pain)? Maybe everyone should beat time like Sacred Harp singers, at least in rehearsal; it would help them internalize the beat.
In rehearsal you will need at least a facilitator, and actual direction/making artistic decisions is more efficient.

Another thing: read, read, read! Don't spoon-feed notes! If you need 2-3 on a part at first for security, do that. But they will only learn to read by reading.If they're volunteers, you can pay them in repertoire...which means doing enough repertoire to be adequate pay.

Oh, I see where the confusion comes from--something dropped out from what I first wrote. This computer is jumpy. My guitarist has dragged his heels on the repetoire I picked for US to do in bars, etc, for fun. I finally paid for a singing lesson for him so he could understand why instrumentalists and vocalists can REHEARSE together, but can't PRACTICE together, if that distinction was clear. He works with a wonderful Brazilian singer, and I pointed out to him that she just sat there why he and another guitarist worked on the arrangement.

To get him to actually get with the program, I picked his favorite jazz song--Bernstein's Some Other Time--and showed him how an octave drop is a killer for me but not him. 'Play two notes on guitar. Now listen to my voice as I sing the same two notes." So he doesn't want to spend a week listening to me going, "Oh, well." (If you know the song.) That was what I meant. He is not doing Palestrina. He is doing a reggae transcription as I write. But his reaction to simple music in our repetoire---exactly because he is stuch a stellar musician--is what got me to thinking about repetoire FOR SCHOLAE. Two separate repetoires. My mind jumped the way my computer does. Enough of me now.