I would allow around €5,000 - €6,000 to sort the car out.
The engine is not complicated, so not much can be wrong. The cylinder heads are expensive, but your problem sounds like the ignition timing or the fuel/air mixture is wrong.
You won't find a later US spec left hand drive car.
If the metal chassis is not rusted, it would be worth buying at the right price. What it is worth to you when it is finished is for you to know. It all depends on how much you want a Europa Special, and what you would have to pay for a really good one. To get a good one from the USA and bring it to Europe would cost you $24,000 = €17,650.
Therefore I think it is too expensive at €17,500. That is a very "full" price. For me, that would be top price if everything "va bene". Offer no more than €12,500 to allow for sorting out the engine and other things. If he does not sell, buy a good car from the USA and ship it over.