Most years of that gen Monte have something special about them that makes them imo worth possessing if you really want one. The one and only thing that is special about the '85s is the interiors. It is the only year that they produced super plush crush velvet seats, and many of them also have a thin lined pattern in them, also. The only downside to that is that replacing them is not easy if you want to keep it original because only so many were made and interiors tend to get really worn.
A-pillars rotting is not common. What most likely happened is that someone replaced the windshield some time ago and didn't do it very well...so water got back there behind the glass and started rotting the steel...which escalated to what you've seen. You can find replacement A-pillars to weld on if the rest of the car is in good shape, but to do that, you would have to remove the windshield and remove the dash.
Big rust issue spots to look for are hidden under the carpet. To really determine, you would have to pull out the front seats, remove the carpet strips under the doors, the b-pillar trim behind the doors, the kick panels, and then peel up the carpet to look under the foot space on the driver side and the passenger side. You could also look carefully at those spots on the underside of the car if you can't do that.
The doors are pinch welded along the bottom, and when the window sweeps fail, debris and water build and rust out the doors along the bottom. The pinch weld at the bottom of the trunk lid along the back does this too, but usually not as badly as the doors. The areas inside and outside around the rear wheels is also a good spot for major rust. If the seams around the rear wheel wells inside the trunk don't have rust holes, you can be reasonably certain the car isn't a rust bucket if all the other areas are okay too.