Well, rust is going to be the biggest concern. Check the strut towers in the front, from the inside if possible. Also check at the front of the inner fender, that is also a common rust spot. Check the rockers, both underneath the car and on top of the rockers under the door sill. The rear axle mounting points are also prone to rust. While you are under there, check the rear axle closely as some of them were flawed and can crack where the "control arm" that the stub axle mounts on meets the horizontal beam. Check the cowl area below the windshield for rust, that's a bad spot, and if it's really bad, it can leak water on the fuse panel. Chances are, the door seals, window scrapers, etc.. are all toast, there are replacements available, but look to spend about $1200 for a complete set of seals. If it's really an 'S model, it should have the Zender front airdam and aluminum-look dash and sport seats.
Oh, and be SURE to check the front crossmember, that the radiator sits on and the front engine mount bolts to. They are often cracked there- it's repairable but it is also evidence that the car has been run pretty hard and chances are it might have front frame damage.
Otherwise, there is probably a %90 chance that the suspension will need a complete make-over- control arm bushings, ball joints, tie rods, strut mounts, etc... pretty common for A1's really. But that's cheap and easy stuff compared to body work.
Dont let any of that deter you though. If the car is overall solid and the price is right, snatch it up and save it.
Brendan