1990-93 Bridgestone MB series is kind of a sweet spot, the MB2 and MB3 don't fetch top prices but still have halfway decent Ritchey Logic steel. Catalogs are on the internet (eg on Sheldon Brown's site) so you can read (at length) about the design philosophy. Which I think lends itself to mixed use. If you happen to stumble across an XO at a decent price, buy it stat, no matter how ugly it is.
Kona has a really nice ride, that would be my choice for dirt, maybe not quite so much for pavement unless you're looking to build a hooligan bike. Lava Dome has really decent steel but (like the MB3) can be had for a bargain price sometimes.
Personally I feel 80's mtb's are great on the road, a little heavier if that bothers ya. Go a size up for more of a roadie feel. I'd love to have an old Schwinn Cimarron although they aren't really easy to find. But something like that. If you come across an old Kuwahara, buy it
Nishiki is kind of a sleeper. I just acquired a Cunningham-signature Cascade for $50, an interesting frame that strikes me as a great candidate for gravel conversion (a beautiful example pictured
here).
Other "sleepers" -- Believe it or not, mongoose and diamondback had some very nice to super nice race frames bitd, dunno if you want "race" but there's some fine steel to be had, just gotta do your homework so you don't come home with a piggie
Shout out to Trek, their lugged frames make great roadies, the later 90's TIG frames are super, super nice off road. There's not a huge difference between the different 900 series frames, they were mostly varying components so if you're gonna mod it anyway, you can save some money by buying a 930. As a bare frame it's a great "blank slate"
I've never got around to buying a Spesh, they just strike me as overpriced typically. The ones I'd be interested in aren't likely to be in your budget (or mine). Same goes for Gary Fisher generally, I kinda have a soft spot for em cuz it was my first mtb but again, kind of overpriced except for the ones nobody can afford