I started my career as a pin ball repair man, about 4 months ago. I bought a 70s vintage Gottleib "King Pin". I still haven't gotten it completely working, but I have found it enjoyable.
I hate to give any advice on this... since I really don't know much. But... I think by the 80s , they were using more solid state components, which may or may not be harder to work on, than the 70s and earlier ones, where everything is mechanical/electrical.
I've heard that you should buy a fully functional machine as your first... I didn't heed that advice, and I'm sitting with a machine that I can't play... I could not afford a machine that was working, at least not one that I liked the theme of, etc... I hope to get back to working on mine this winter... but first I need to get it out of my garage, and down into my basement where it will be warm. ( need to do that, like soon too ).
Oh, one last bit of advice... Make sure the play field is in really good shape... it's very hard to fix if it's not. The back glass is also something you want to pay a lot of attention too, as this is also a item that's very hard to fix , and for value... the back glass is one of the most important things. Also... don't be in too much of a hurry to buy your first, do your home work ( again, I didn't follow this advice , and I'm a tiny bit sorry ).
Anyway... good luck.