First consider that my opinions are tainted by living in a place where everything rusts pretty quickly. I'm going to assume you don't have it as bad in Colorado.
Since you said 88 GT I guess you have done your homework and have found this is the newest and most advanced (suspension wise) model.
It's a 1988. Everything applies that does, if you were looking at a Caprice or Mustang. Rust will first appear in areas where hidden dirt and moisture can build up. The area around the battery, the sides of the trunk (pull back the carpet) and the rear frame rails (under the fender liners) are the worst and first places.
Get the best and most complete car you can afford. Some parts are now harder to locate and some trim may be tough to sort out. If something is missing it may be because the owner can't get a replacement. Cars that look like an easy fix can turn into money quickly. If it has been sitting a while, brakes clutches, suspension parts may be toast.
The good side is that most common stuff is of the shelf GM and found on other cars.
Don't jump at the first 'deal' to come along and read the forums at
www.fiero.nl for tons of info.
Gary
Author