Ultimately any used car is a risk. Personally I'd casually ask the previous owner what fuel they used, and depending on their reply I would walk. If buying from a garage you don't have that luxury, so you just make sure that everything else on the car is fine and go for it.
I don't think you're worrying over nothing at all, your concerns are definitely valid, but as there's nothing you can realistically do I say just take a gamble and enjoy a great car.