First, take her out for sea trials. While out on the water check out the function of everything, the head, the radar, the furnace, the wipers, the fridge, the stove, everything. A surveyor will probably check that there is power to the electronics and appliances, but will not check that they function properly. If you still like the boat and decide to make an offer make it subject to the results of a marine survey and mechanical check over.
Yes, the marine survey is worth it, you'll need the survey for insurance purposes. Your survey will state any deficiencies and safety issues, which the insurance company will likely make you correct. It'll also put a value on the boat which will determine how much a total loss payout would be, and how much your premiums will cost. Remember, surveyors are not mechanics, so you'll also need all the machinery checked out. I see nothing wrong with using the dealers mechanical dept to do this, as long as your comfortable with their reputation, and they provide a written report of their findings.
As others have pointed out any fuel onboard will be spoiled, and will need to be replaced. If I was buying her, I'd have all the fluids changed, as well as the belts, hoses etc.