Emptyjar, I too am a fan of the older road bikes having restored several, some that I continue to ride on occaision. However, with mountain bikes, it depends on your application. I ride a vintage, 88 Nishiki Ariel on gravel MUPs and I have a 92, raised chain stay, Nishiki Ariel that I've ridden on single track. But neither of these bikes can compare to a late model mountain bike on hardcore single track.
Also, with older MTB's be careful swapping a rigid fork for a suspension fork. I've made this swap several times with mixed results. The 88 Nishiki came out as a really poor handling bike after the swap. That's not a problem on a flat straight rails to trails with no obsticles, but on a single track MTB trail, it was terrible.
If you're going to go on single track, MTB trails, go late model and save yourself the learning curve, however, you might still want to try some used equipment. I'm currently riding used Stumpjumpers, an 08 hardtail, and a 07 dual sus.