My 1994 BMW 318i, replace head or get new motor?

Is it cost effective to replace the head gaskets or just get a used motor? I was advised that gasket replacements for these cars is especially difficult. There is water in the motor due to overheating. I procrastinated in replacing a leaking radiator. Now that I replaced it there is a small hissing bubbling sound coming from the block. Gaskets? The head itself? Please help!! I live in the San Diego area. Is there any places you know of that sell used motors or does this cheap??