Its a tough one. Obviously you want your engine. But at the same time, you don't want to get scammed. Perhaps you rushed in a bit and should of asked for engine numbers, more pics etc.. sort of got a feel for the buyer.
Hopefully if it goes pear shaped, you will have PayPal buyer protection as it is an escrow service and both buyer and seller should be happy before money is released.
Cheers,
And good luck with the project..