For $80, even a YUGO is a good deal! Well....ok....maybe not a YUGO, but you got a decent car there. Even if it costs $1K to get every single thing on it running tip top again, $1,080 for a smooth running car is a bargain. But before you go throwing money at it, I'd have the FPR checked first. It could be that he just told you it'* the FPR to make the problem seem simple, or get questions of why it'* running rough out of his hair. Also, while the upper plenum is off to do the FPR (if indeed it'* bad), I'd have the injectors yanked off, and cleaned/balance flow tested. Those FI treatments in a bottle really don't do jack, quite frankly, except collect the deposites, and sometimes clogg up your injectors, making things worse
Rotors and pads are cheap enough. I'd just make sure the rears calipers aren't locked up. '88 - '93 W bodys are NOTORIOUS for this. In fact, GM settled a class action lawsuit for them a few years back to the tune of $19.5 million!
Unfortunately, it wasn't an all out recall. Just reimbursement for past repairs through a certain deadline (which has LONG passed). Upgrading to the '94-'96 style w-body brakes is a pretty easy mod, and a LOT more reliable (completely different design rear calipers).
Other then that, it'* solve one problem at a time as they come. Congratulations on your purchase! Good luck with it.