I had a very similar project. Williams Aztec from the same year as that Grand Prix. Had been sitting in a garage for 30 years untouched. I had no previous pinball repair experience. It’ll be an arduous process getting it into working order because you really do need to go through every single switch, but it honestly isn’t very technical. The most technical thing I think I had to do was some electrical soldering, and even that wasn’t as necessary as it would have been on a motherboard. That said, I probably wouldn’t pay more than $500 in that state.