^^^^ This. Even if you fired with a wet engine no radiator (and he let you) it won’t give you a full idea relative to what is wrong. You could read the plugs and borescope the cylinders but that still isn’t going to get you that much closer. Unless you can hear it and step on the gas a couple of times to see if it sounds like a canister of Lincoln Logs it’s hard to tell if it’s impending doom.Just low ball your offer by the cost of a complete engine rebuild. Tell the seller to get a radiator in the car if he wants you to up the offer.
If you had your location listed maybe another member here could help you out with a loaner.I'm thinking an engine problem is probably being hidden as well.
I like the idea of running it with water to get the engine warm and running a comp test. I just didnt want to buy a radiator for a car i might not buy. Thanks for the help!