I am in the process of setting up an old GT Karokoram as a single speed. Got the bike for $10.00 from a CL listing for just the frame. It is a well used bike, but the frame has no dents and looks to be solid. The drive train is worn, so I decided to take the rear cluster apart to select a still usable sprocket, and set up a quickie SS. The rims are worn, but still have a bit of life left in them, so I it seemed like a reasonable bike to experiment with.
It has working Deore thumbshifters (6 speed) that are going to go up on ebay.
I have another GT, an Outpost that is in nice shape, except for 2 totally non-working rapid fire shifters. It also cost $10.00. I tried the flush with WD-40 (which has worked wonders on other rapid fire controls that have been trouble) but in this case -- no help. I have a set of friction thumb shifters that I am going to set it up with. It should make a nice campus/around town rider.