is the Mercury 502 EFI/MPI a good base engine to build up on a mid nineties Top Gun? what needs to be modified to make it durable if blower is added?

thanks

Yes. Some recommend swapping out the stock head gaskets with blower installs...my experience with the Whipple units is the stock parts work fine.

Go with Fel Pro 17048 head gaskets. The exhaust valves should be inconel-Manleys or Ferrea. I sent my heads to Teague for the valve job, springs, etc. Make sure the crappy Mercruiser cast exhausts and elbows aren't leaking. Otherwise, stock exhaust will be fine.
Also, on a Top Gun, you're going to have to upgrade the Bravo's. Imco Extreme, Max Machine Worx, Teague XR are all good options.