The wheel size debate is interesting and fun to talk about...but I have to say I really don't see plus sizes being anything more than a niche. A fun niche, but still a niche. Kind of like the all mountain hardtail niche.
OP, I have a 26" hardtail that is from the mid 90's that I've been accumulating parts for a build and I would strongly recommend not getting a frame that old to build up. What you run into besides for geometry differences is that the brakes are different, the rear spacing is different, through axles weren't on the radar then, 80mm of travel at most and you are stuck running 26" tires. Not that I'm not going to finish building this bike up and riding it until it breaks or my smiles go away, just that there are better modern options.
I would be choosing a 29'er if going hardtail or shorter travel FS, or a 27.5 if going for a longer travel bike. I'm a relatively short dude and still really like 29'ers for speed...the 27.5 is fun to turn, whip around, jump and the ones I've ridden have all had a decent amount of travel and have been really fun on the downhills. I see a need for both, honestly.
If I were shopping for a frame to build up today if new or used, I would for one be making sure it uses disc brakes (and most modern ones do); two, that it has a tapered steer tube; three, that it can accept modern bottom bracket standards (and honestly, IMHO, threaded is the bomb); four, have a wide enough seat tube to accept a dropper post like a reverb and internal routing is great for that; five, it has the geometry for the riding that I intend on doing with it. So, if more of an xc rider on relatively smooth flowing singletrack, a steeper head angle would be fine, but if I plan on doing jumps and downhills a lot I'd be looking at slacker angles. I would also be looking at getting the appropriate amount of suspension for the riding that I did. Around me, a 160mm bike is fairly useless where 120mm is wonderful.
Good luck...it will be fun.