Rust is the biggest risk, IMO, with any old car - and it can be hidden expertly, Cars that spent a lot of time in northern states have been subjected to salt, and old cars just couldn't deal with that. Everything from the frame to the firewall need to be checked thoroughly. I would get a good look at it on a lift, if I could. If not, crawl around under it as best you can - look in the wheel wells, check the floor pans for holes, etc. Wherever it is painted - outside and under the hood - look for even slight bubbling/ripples. People can do amazing things hiding even catastrophic rust. If it gets into the car too deeply, you're looking at tearing it completely down and looking for a metal fabrication guy. If it's just in the fenders/quarter panels/doors/hood you can replace/repair those.

I would also try get somebody to drive it, while you follow, and see if it rides straight. Sometimes you can notice a bent frame from an accident - it looks kinda like a crab rolling down the road, sorta sideways.

You can push down on it on all for corners when it is parked and see how it responds. Does it really give a lot? Does it squeak loudly? The front end especially, can be pricey if it needs a lot of work. Do the usual stuff - check all the glass, roll up and down the windows, open/shut everything, check all lights, fans, locks, etc. And smell it! You don't want mildew smells.

The seller is a mechanic, so see if you can get an idea of his reputation. And ask him point blank what still needs to be done, what he had planned to do, and why he didn't do it, etc.

All, and all though, 8k is not a lot to pay, so you really don't have a lot to lose. I wouldn't think much of it is original, nor would I care, and reproduction parts are easily available if you want to work on it from yearone.
Sounds like it could be a great thing for you and your son - I'd go for it.