Should I still have a diesel surveyor evaluate the engine, in addition to the surveyor who is checking out the rest of the boat?
For the investment you are about to make, it would be silly not to have an engine survey done. It would cost less to have an engine surveyed, than to buy it outright later to find that the engine has issues, then pay a mech. to track down and fix the problems. If the survey is clean it will be a big "what if" that you don't have to worry about.