I think Sebach's suggestion is a good one. You can get a surly frame from spicer for 350 US+ pretty reasonable shipping. I think the 26" wheels are an advantage, not just due to size, but overall. You get a stronger wheel IF you go for 36 spokes, and you can get parts anywhere. You still need to get a really high quality wheel build. It isn't difficult to do yourself, or find a shop that really knows it's craft.
I was walking around TO today, and noticed a number of abandoned, stripped bikes with pretty good old MTB frames that would make a bare bones touring bike, when restored, probably a police auction or something would be even better. Also, just keeping your eyes ope prior to garbage night, and if you have a while you will find somethng useable. But if you can get a decent frame and fit off the rack, why not.
The basic frames to build out (you can just switch a bunch of components off your MTB if you want), are the SUrly, The Urbane tourist, and the Nashbar. They all come with good riding forks.
If you want to go a little upscale go LX hubs with velocity rims (or any better level hub like phils). Or you can get by with something like the DH22 rims by Alex. Get 36 spoke. Drops are the ticket, you can use flat bars but it's not ergonomically correct. MTBs don't have flats because they are better riding bars than flats, but because they are more powerful wider, and I think probably a lot better for stuff like riding over logs. But for grinding out 5k Mi.
You will probably have to pay a bit of attention to the brakes. A road bike can run the exact same drivetrain as your MTB, but nobody makes a touring brake/lever set. Brakes don't need to be expensive, check out some recent threads, but they need to be carefully paired up, foirks, levers and crakes need to be a match.