First question is "how much are you wanting to spend on a 1994 truck?" Putting an OBDII engine in it would be pretty expensive and would offer little advantage over what you could install that is non OBDII.....OBD (on board diagonistics) monitors the sensors and reports conditions that are out of range for the event. Why do you need it?

You could install an el cheapo rebuilt engine...or rebuild the existing engine. Either would reuse the existing fuel and electrical systems.

You could install a GM crate motor....a true "crate engine" not a rebuilt engine that is mistakenly called a crate motor. The GM engine is all new. There are many versions to fit the cost and horsepower needs of the buyer. If your existing truck is a TBI truck, you can install a carbureted engine relatively simply....you need to add a fuel pressure regulator to drop the fuel pressure to 6psi or so. Do a search for GMPP or GM performance parts and see what they have. Summit sells GMPP crate engines I think.

I would recommend not buying an engine that the mechanic is pushing unless you can have an independent third party look at it closely....The mechanic seem to be looking out for his buddy with an engine for sale....not necessarily in your best interest.