Don't hold your breath. So unless by some miracle you find a car with every service receipt it ever had you need to start to categorize items into groups for replacement on old cars. Everything has a life so you need to determine what those are. Even with the receipts it may not indicate what work or part was actually replaced. Some shops depending on brand can get access to dealer service info , but again it just may not be available. So potentially you may have no status on several items. If what you buy is fully running and functioning I would set up a two year plan to replace all fluids and all the lines and rebuild all the systems attached to them. Since it is air cooled you don't have water lines and coolant radiators, but those had all kinds of oil coolers all over the place(which are sometimes worse than the water ones). Those have possibly 25 year old lines that have possibly broken down with time so expect to figure out when to change all that. Expect to spend several thousand to buy new plug wires and plugs for this car and that was what they costed about ten years ago. All the braking system lines that are rubber should be replaced and all systems flushed out. While doing that you probably want to rebuild the master cylinder and all the calipers. Also you probably want to put in new rotors may be in order and while doing that put in new bearings. The half shaft boots may be close to cracking and the axle bearings may need replaced and should be in the 3-4 year plan if they do not fail before that. Then there are electrical and other rubber things like window motors, fuel pumps, alternators, gas lines, ABS computers, engine computers, door seals, the rubber guides for the windows in the door, shifter boots, cracked dash and a thousand other thing to fail (remeber this is a cutting edge sports car when it was made so there is a good deal of tech here). Also remember this vehicle has had 25 years for the everything in it to age and decay. Murphy will come visit if you try to push this car and do not go through it throughly before taking it for more than a 20 mile drive. If you don't have towing, get it and know how far they will tow something back. Even with good reliability you can't battle age.
It took me four years and double what I paid for the car to get my 2002 XK8 to the point that I'm not afraid to take it on a weekend trip to the Keys(about 4-5 hours). These are fairly reliable, but you learn pretty quickly that even the best and most reliable cars loose to age. About two weeks into ownership the roof lines blew in the a windshield header spewing green oil down. Google green shower on a XK8 forum and nearly every member has a story. You never find these things until it happens to you. I'm certain there is at least a list of 20 things that break quite regularly on an air cooled Porsche. Even Toyota, Hondas and Subarus have these lists and they are near legendary for reliability. You can't win against time. I do 60% of the work on the XK8. Finding a Porsche that has all these things addressed before you buy it will cost more than a 4 year old new one. So that was why I did what I did and those were only 14 years old when I did that. Generally pristine Porsches that are not on the Mecum auction block are up for sale because the next round of spending is about to start. Generally the people that keep them are mechanics with good access to parts, or have money like Jerry Seinfeld to spend on them to keep them running.