The down side of the full size cars is parts availability. The A-bodies have far more aftermarket parts available than B-body cars. This makes parts harder to find/replace if needed and prices are typically more. I assume it runs and drives? You did not state how many miles. Top works? Heater/controls work?
Like any car, you want to get underneath and check the frame to make sure it is solid. Have a flashlight in hand. If you can get it on a lift, all the better, but if not, jack it up and secure it with jack stands and crawl under it. Check those areas where the frame is boxed in (which I supposed most of it is being a convertible) and up around the rear axle - anyplace where water might accumulate and sit stagnant and cause any rust outs. Peer into any of the larger stamped frame holes with the flashlight or feel around for any heavy pitted rusting.
Check the flooring and inner rocker boxes, inside the rear wheel wells at the bottoms where they often rot away. Check the inside lip of the rear wheel wells where they meet the rear quarter. Use the flashlight to look between the rear bumper and the trunk body panel - if possible. Look down into the front fender corners with the hood open looking for any heavy rust, rot, repair work.
I suspect it is like any car purchase, give it a once over and use that flashlight to inspect as much of the car as possible. Check all rubber gas lines, brake lines, hoses, etc. for dry rot/cracking. With car off, have someone turn the steering wheel a couple inches each way while you observe the front end, ie idler arm, tie rod ends, cross link, steering box etc. for wear/play. You have no way of knowing the condition of the drivetrain unless something has been rebuilt with receipts. All of this can get expensive the more work you have to do to get the car in a safe & dependable running condition.
The 9K price tag may or may not be a reasonable deal based on what you will have to put into the car and what you plan on doing with the car. Just like any car, you could put more money into it than it is worth should you try to resell at some point. So you just have to be honest in what your budget will be depending on the level of repair/rebuild you are looking to do and not what your Pontiac heart strings are saying.