Julie, is this your broker (i.e., buyer's rep) or the seller's?
I don't think it's unreasonable to ask the broker to start the engine while you're there to see the boat. Asking to go for a ride may be a bit more of a stretch. As has been said, you get that chance during the survey and sea trial.
Have you checked with the folks at Choosing an Ericson 38?
They may be able to point you to some E38 owners in your area.