I'd start with the head gaskets too before going down the road of swapping the entire motor. 160k is fairly low miles if the engine has been cared for.
I tend to use a can of Seafoam in with a full tank of gas every other oil change. I don't expect any miracles, but figure it can't hurt. Even better when I can catch a can of it on sale.
I've run Amsoil in just about everything I've owned and cared about. In each instance, something other than the motor has forced me to parts ways with vehicles, thanks rust!
With any new (used) vehicle purchase, I change the oil, run a 3000 mile interval and send a sample off to Blackstone Labs...