Sea trial is obviously best; however, I would not necessarily walk away. But two motors, drive's, air conditioner, gen(?), etc. that can't be tested under load - there is a lot that can go wrong.
A sea trial is best, but a thorough examination of the motors - compression test, leak down test, vacuum test, should indicate if there are any concerns. Perhaps in lieu of a sea trial you can negotiate down on price to have the engines examined (and re-winterized) and then a little left over to cover anything else that comes up?
Twenty some years ago I bought a boat in December. We did launch the boat and sea trial it, and ran the generator - but didn't test the air conditioners or pressure water system. Care to guess what didn't work after I launched my boat in April?