Most MTB frames for 26" wheels have something to offer in the way of touring conversion. There were several distinct phases of MTB development (not all of which were adopted at the same time by all manufacturers) and each phase has different implications for a touring conversion.
A) steel, LWB, slack angles, horizontal top tube, until about 1989.
B) steel, shorter wheelbase, sloping top tube, until about 1994.
C) steel or aluminium , sloping top tube, larger size threaded headset, geometry set up for ~60-80mm travel fork, until about 1996.
D) steel or aluminium 1-1/8" A-head, possibly with longer top tube, geometry set up for 80-100mm fork, until 2000 and something.
A) is probably the best geometry for carrying a big load, simply because of the wheelbase. With a shorter wheelbase you can't use the biggest rear panniers; without front load the whole bike may not even keep it's nose down unless you are sat on it. But the strength of the whole thing may be limited by the 1" steerer. Note that A type frames will usually be 126 or 130mm at the back; the change to 135mm occurred later than many people think.
A,B,C are the ones I would look at for use with dropped bars; D often has a top tube that is too long to make this practical.
Steel is probably the material of choice and if a bike comes with suspension forks you will probably want to ditch those for steel rigids. They need to be correct for the geometry of your bike though; if it doubt measure the crown height which gives you the correct seat angle you want. It is much the easiest thing if the bike comes with rigid forks to start with; some C type bikes used oddball headsets like 1-1/4" threaded which is a bit problematic nowadays; if a new fork is required it is probably best if you use converter parts to use a 1-1/8" fork and headset.
A common problem is that folk want to use low-riders on a fork which lacks the correct mountings. I happen to think using higher mounted panniers isn't that bad after all and if you want, you can have mountings brazed onto a steel fork easily enough, so it isn't that big a deal.
MTB gearing is usually about low enough (or can easily be made that way) but a common problem is that if you ride briskly on the road at times, a ~42T big ring may not be big enough; you may end up using a 14T (or smaller) sprocket on the flat, which is both inefficient and fast-wearing, if it isn't rough-feeling all the time, that is... For such use a 46T or 48T big ring makes more sense, even if the intervals between chainrings end up being rather larger if you have low gears as well.
MTBs tend to come with 175mm cranks and relatively high bottom brackets. If you fit more 'road' oriented tyres they are usually narrower and the BB height comes down a little. I prefer 170mm cranks for road use, and if you are swapping cranks anyway, you may as well get the chainrings you want too.
Another thing I don't much like is a large 'Q' value and normally this remains uncomfortably high on any converted MTB. In fact this is the main thing that puts me off using such bikes more than I do. I guess I have owned/converted/used between fifteen and twenty such bikes in total.
An upcoming project of mine is a ~1988 Dawes MTB of the type A variety, in 531AT tubing. The frame is pretty beat up-looking but still sound; I intend to build it as a 'beater'/utility bike, but a lot of the priorities in the build (load lugging, strength, repairability etc) are pretty much the same as they would be in a touring build too.
For wheels you can still buy 26" sputnik rims and these make for excellent touring wheels.
cheers