my
It would be hard to build an engine for $5K with what this one supposedly has in it. I personally would do the following;
1. go and see the engine for yourself and talk to the owner.
2. try and get some type of guarantee that it at least isn't knocking or otherwise bad
3. negotiate as low a price as possible
4. buy a gasket kit, take it apart before installations and inspect the internals (make that a part of the guarantee; that nothing is broken and it is assembled correctly; just pull the heads, drop the pan and you should know/see everything you need to, maybe pop off one or a couple of bearing caps)
5. reassemble, install and have fun