I actually bought a tenor banjo from a guy on eBay - the transaction was fine, got the banjo, wasn't too disappointed considering I didn't expect too much. I bought it with converting to 5-string in mind. It was a cheap old Slingerland and the fretboard was dried out and cracking all over, but of course that's one of the parts I'm replacing anyway. Bottom line is that my experience was positive. Be sure to read the seller's feedback. There are many tenor banjos there, at good prices.

I don't know much about tenor banjos but I'm guessing the ones with 17 frets have a shorter neck than the 19-fret ones. I've never seen one with a fretboard extension over the body although that seems to be standard on banjo-mandolins (which are double strung and tuned an octave higher).

By the way you can play Irish melodies on 5-string banjo. There's a book w/CD by Ken Perlman that tells how (published by Mel Bay). I've learned a few from it.