Re: PTR-91
If you are not put off by the terrible trigger, the unreachable safety and mag-release, and the lack of last shot bolt hold open, they're accurate enough rifles I guess. I ditched mine for a FAL back in '05 or '06, but from what I remember it was an honest 2 MOA with quality ammo.
Best thing about them is the cheap mags. Don't know what they're going for now, but there was a time they went for about $2 a piece.
Scoping it sounds good. I scoped mine. I don't remember what the mounting options are anymore, but I'm sure someone here does. If it was me, and I was doing it again, I'd go for some kind of low power variable, a good 1-4 or something along those lines would play to the rifle's strengths better than a red-dot or a full power scope.