if you can only afford to buy something and not install it, don't buy it. if you can't pay for it to be finished, you can't afford maintenence and repairs. if you can't afford maintenence and repairs, you probably have trouble paying the light bill.There's nothing wrong in buying the engine and having the patience to put it in later on. In my opinion, patience pays off and is a virtue that should be followed unless you want to rush into something you either can't finish or end up screwing up. While it's sitting there looking pretty you can also buy more parts for it and end up with one hell of a beast. haha What engine do you have now? If it's a Z6 or Y8 you could have just a little bit of fun with it while piecing together your new build. Just my two cents. Good luck with your decision either way.
wait until you pay for the whole thing.
read: unpainted body kits installed on cars.