MISERERE Antonio Lotti; despite getting your men to nail the minor second initial dissonance that's immediately resolved, this is not that difficult a piece.
O BONE JESU G.P. da Palestrina
ADORAMUS TE CHRISTE Palestrina

Adoremus Te by Dubois from the Seven Last Words of Christ.

Some of the above suggestions require treble voices. If the fledgling schola is limited to men (or for that matter is limited to treble voices), consider the Ravanello collection of motets for three equal voices. You'll find a link to this collection to the right on the home page and several pages down all by itself under "Choral Music".

Most or all these arrangements are transcriptions of works written for full choir, but inasmuch as they are for equal voices, the range is not too demanding. A purist might object to not singing the real thing, but this anthology has a lot of beautiful and comfortable music for a choir of limited facility.

Sorry, Prof. Mahrt.....attributions in these forums are generally for brevity's sake, as opposed to the precision required when completing masters and doctoral degrees. I'll raise my goblet of Pinot Noir toward Stanford next time I hear "Bach's" BIST DU BEI MIR. ;-)

Yes, that De profundis of Gluck is wonderful, that was our choir's "big" new work last Lent. A good change from.... well, I won't say what their favorite Lenten anthems have been. They are starting to have a little more faith in me and confidence in themselves.
For English, there's a very nice O Faithful Cross by Englert, IIRC.
Richard Proulx's setting of the Canticle from I Peter.
And for the 5th Sunday of Lent, Croft's I Am the Resurrection.

(Save the Liturgy, Save the World)

Also, I just found O Jesu Christe by Melle. Can't find a recording, but it looks very doable for a small choir.

http://www.mipucpr.org/cantoral/wp-content/uploads/OJesuMelle.pdf

Update: CPDL to the rescue again! Found a relatively simple but dramatic Stabat Mater by Juan Frances Irribarren (1698-1767)

Score: http://www0.cpdl.org/wiki/images/sheet/iri-stab.pdf

http://www.youtube.com/watch?v=cxA2dAGEKEk

Also, don't discount the value of singing Lenten hymns in 4-part harmony.

Tangential question. I am trying to distill some singers from my choir to dedicate exclusively to chant. This will require a bit of extra rehearsal of course, but I'm sure that my folks will enjoy it. I have truly gotten a few good pieces to teach from your suggestions here, but since I am in the formation stage, is there an optimal number of singers that I should seek to assemble? 

Wow! I love it, Charles. It seems like Lotti was an incredibly diverse composer. Everything I've heard by him sounds completely different.

I'm not having much luck finding a score. Did find this setting of an Adoramus Te, Christe in G minor by Lotti, though:

http://www.ancientgroove.co.uk/pdfs/lottiadora.pdf

This has been the most inspirational thread I've read all week! We have 3 strong voices (including myself) right now as a fledgling schola. Can anyone recommend some pieces for 3 voices? I remembered I had downloaded "In Monte Oliveti" by Martini, and that will be coming out for Lent! I'd love to hear some others though!

Barnabus, which three voices? SAT, ATB, ATT, or what?

I recommend:

God So Loved the World (from The Crucifixion) by Stainer.

Ave Regina Caelorum, the chant followed by the motet for three voices by Dufay.

Many thanks for drawing attention to these, Julie. I think I had encountered the O Salutaris before, but the others were definitely new to me, and I'm particularly taken with the Fux.

Thanks for the recommendations! I'm focussing mostly on 3 equal voices, or TTB. I can probably adopt any arrangement for 3 men, but I'll go through these and see. Thanks again!