Cyccommute knows his medicine.
But, I would say that his cautionary advice on old MTB's might be tempered by these words:
Many mountain bikes from 1986 to 1991 had vitually all the right hardpoints to attach racks and fenders. They had slender, tapered touring type forks - many with mid-fork rack bosses (this very desirable feature began to dissappear by 1992 and later). Long wheel bases were the norm, like a touring bike. Long chainstays were also common, another touring bike essential. The gears were perfect for general road touring and most hills, with the excellent and robust Shimano Deore or Deore XT groups (now only considered entry level).
In short, and if it's all in decent shape, there is not that much to do to prepare the bike to go. Just add the racks, lights, fenders, a comfortable saddle, good tires, and maybe, maybe different handlebars. Only if I weren't pleased with the stock bars would I bother with that, although drops are the preferred bar set. Lots of people tour with a variation of the straight bar. Most all of them will accomodate your stock brake levers and shifters.
Without the drops you can easily outfit one of these for under $500, or, half the price of many new bikes. And, I maintain that a good quality MTB is a stronger bicycle all around than most touring models.
I agree that 83 to 91 mountain bikes are probably the best ones for touring (I'd go back to the original Stumpy
). However, just like land, they are making them anymore. And the ones that were made weren't quite up to the standards of construction and ruggedness of today's mountain bike. Many of the oldest ones were ridden, literally, into the dirt. You really need to be extra careful and inspect the bike thoroughly for evidence of hard riding, i.e. cracks and gouges. If you find a good one, by all means make a go of turning it into a touring bike