Advice above about bringing someone knowledgeable is at the top of the list. Keep in mind that no matter the car you buy it WILL break. It is a good idea to source out the Turbo Buick guys in your area. Generally speaking they are great guys and typically will lend a hand checking it out for dinner and beers. They love these cars and welcome seeing them at every opportunity. Finding one in your area, willing to go with you to check out a car will help you a lot. They can generally spot problem areas and highlight problems which are 'no big deal'.
I spent 3 years looking before I bought mine so my personal advice is to take your time. Don't let your excitement get ahead of your brain. You are buying at the end of the 'driving season' so a lot of cars come up for sale around now so there are some deals to had, but the wrong car will be a heart break and a money pit. You need to buy the best car you can afford. Highest price doesn't mean the best though. A single repair can take $750+ out of your wallet. I preferred to spend that money up front with a nicer car. I had a price range in mind, but I always asked myself 'What would 1k more get me?'. There are lots of things to look for. When I was looking at cars, I started at the cosmetic level and worked my way down.
Your first enemy as Dennis pointed out is the odometer. they are only 5 digits. so 110k looks the exact same at 10k. Keep in mind there is nothing wrong with 110K on a well cared for TB, but it should be disclosed and reflected either in the price, or documented in maintenance to over come it. So you will need to identify where to look for signs of high mileage. (Wear on the steengwheel/pedal/carpet for example). Folks might disagree with me, but I hold the opinion to NEVER buy one of these cars from a dealer. It is amazing to me how every deal that has a TB that was driven by a 'little old lady' who only drove it to the grocery store for 35k tender miles. Really? She upgraded the Fuel injectors, turbo and installed a Front mounted intercooler to go to the grocery store? (And expereince TB guy could identify those things)
Moreover than the stuff on the actual car, for me it was about finding an owner I can 'feel good' about. I wanted someone that took car of the car, and that it was evident. Everything he says should check out. If he says original paint, then the emblems should all be be in the exact stock locations. If he says he had it repainted, he should have paperwork for it. There are $500 paint jobs, and $20000 paint jobs. Where in the spectrum did his fall. If there is no documentation, then it should be considered unproven and reflected in the price. If even just one thing didn't check out against what he said. I walked away. If he is dishonest about one things, how can you trust the rest.
It is also a good idea to have a 'goal' in mind for what you are doing with the car. If you plan to go to cruise nights/car shows paint and originality could be what you look for. If you are going to be a track rat, performance mods is what is gong to be on your mind. But keep in mind, no performance mod is valid without documentation. It's easy to say it as a roller cam, and ported heads because you are probably not gonna be allowed to disassemble the engine to look. You need to know, was it self installed or professionally installed. Lots of guys here wrench their own cars, but those guys know to keep their receipts. You can generally judge their attention to detail by the state of their engine compartments. Two types of wrenchers out there. Guys that get it going, and guys that get it going right. Both are skilled and good at what they do. Which one's car do you want to own?
Another big thing for me (and folks will disagree on me here I know) but the DATE on those receipts is just as important. LOTS of cars get put of for sale 3 months after MAJOR upgrades were done. That's not enough flight time for me to be assured that the build is broken in and all of the bugs are worked out. There are many cases where funds ran out, life events causing the sale, or any number of reasons. But unless you are prepared, both technically and financially, to deal with potential fallout from an incomplete or improper build, I would generally avoid those too. It's not always possible, but I when i bought mine I wanted two years or more from the last upgrade or major repair. I didn't want to spend my money ping for someone else's divorce so to speak..
If you find a car that is posted online, feel free to post is on this site. Generally speaking nobody here is going to snipe you on a deal, but they will gladly critique it in terms of what they can make out in the pics compared to what is in the ad. It could help you build a list of questions to ask when you call on it.
I hope this helps!