Originally Posted by fotooutdoors

Drop bar or flat bar? Aluminum for rust prevention, steel for ride quality? How burly should it be (what weights do you put on it, do you ride off-road at all, do you ride off curbs regularly, etc)? What sort of tire clearances do you want/need? What braze-ons do you need for racks/fenders?

You are right that not many used bikes accommodate discs. Depending on your location (and hence local CL bike market), though, it may be possible. If you are using a tensioner, any most any disc-compatible frame would work. Personally, though, I think that throwing a tensioner and triple on a a IGH bike sort of defeats the purpose; you almost may as well just go with a regular derailleur system and save some money at that point.

Some places to look for inexpensive frames that get the job done (btw, I am assuming you are in the USA, if not, you will have more local brands that will likely be cheaper):
On-One: shop.titusti.com is their US website. They have decent, versatile steel frames for a good price. Paint job is a bit lacking in durability, but otherwise good. My commuter is built up from their inbred frame.
Nashbar: They have their own house brand, mainly (all?) in aluminum. Not always the best-designed, but totally functional. No options for horizontal dropouts (I know you said you wanted a tensioner, but...). I have a touring bike based upon their touring frame that I have ridden over 10k miles.
Origin 8: They have several inexpensive ($2-300 range) frames that seem decent. Many LBS's should be able to order them via JBS importers. No personal experience.

There are others, but these are a few places that I have experience with or considered when looking for a ~$200 frame for my commuter. Good luck!