I have a feeling the seller is fishing. Having spent $80K in 14 years on a car that was worth $15K or less when he bought it doesn't sound like an astute investment to me.
Replacing speedos isn't uncommon. They break. I had one sent out once by my shop and he asked me what I wanted it to read when it came back.
You've got a TMU on your hands.
Look at all the docs carefully. Shops almost always write down the mileage when they take a car in and you can usually get a pretty good idea looking at receipts. Plus, you want to know what added up to $70K and how it added up.
If all that work checks out I'd offer him the $40k again. It's worth it with all that work already performed. A great car that needs nothing is unusual.