I need to get my Bruno Tiple off my signature now that you have it, Zach. If you look down there, however, I still have one vintage piece left from my days of restoring old instruments. It's an old no name open back Tenor Banjo. Simple - very nice neck, and (now) in all around great shape.
Here's what I've learned if you want to go from steel strings to nylon. First, throw out much of what you know about selecting for wood body ukuleles. You're now dealing with a much different animal.
A drum body produces a lot more volume that a wooden one. Tenor Banjos are normally in 5ths: CGDA. A Baritone Ukulele set-up might seem the obvious choice, since DGBE starts a little higher, and finishes a little higher - in other words, the notes are in the same range. The scale is also the same.
When you move from steel to nylon (two strings wound), however, you lose a lot of brightess and punch. The strings are also much thicker - and banjo necks are pretty narrow. Beleive it or not, I think you get a much better sound going up in tuning - to key of C - in other words, the regular high re-entrant ukulele tuning: GCEA.
You have thinner strings - a better fit for the neck - brighter sound because of the thin diameter, and because you have that drum body, unlike the weak sound that tuning produces on a wood bodied Baritone, you'll still have great volume (though less than with steel).
I think the Tenor Banjo is the ultimate ukulele rythym instrument - it was meant to be a bright rhythm instrument, and it cuts beautifully in that tuning.
There is a group of banjo devotees who like the funky old time sound, with gut strings. I can't use them much here because of the humidity (on the porch, they'll get sticky and stretch so much you can't finish a song). Nylguts (at least the old formula did) give a respectable aproximation on a banjo without the problems in humidity.
Aquila also makes a Baritone set in high re-entrant C. The gauges are here - at the bottom:
http://aquilacorde.com/index.php?option=com_content&view=article&id=96&Itemid=1656&lang=en
The only trouble is that neither these, nor any Baritone set will likely be long enough to cover the extra distance to the tailpiece. To take care of that, you go to Aquila USA, where the classical players buy nylgut. Pick your length:
http://www.aquilausa.com/ng_strings.html
A Tenor Banjo as ukulele, to me, is a great way to go!