IMHO not as much as one would think, only because of that head gasket issue.
I recently turned down a 2 car deal because of blown head gaskets. They were a 94 conti and a 96 Nissan 626 both for $650.
Sure the head gaskets can be replaced for about $1000 each, but you'll never know how much water got into the heads, you'll never know if any water caused rust, and how much rust there is.
I went through the hassle of replacing a head gasket once, and I'll never do it again simply because the engine still blew. I'm only 1 person putting in my $.02, so you can expect more answers that will sway your way.