Dress warm...
Do visual inspection 1st (door can be cut in shrink wrap if needed). Take a few friends- the more eyes the better. Flip on batteries, turn on accessories, fiddle with every thing. Point out potential issues to the seller at this time and what if any thing they can do to correct it. That could be $ off, not worried about it, whatever- just bring them up before moving on to the next step. If you see mold in the upholstery and are going to want $X off for it- if the seller won't budge no sense moving on. Don't have to come up with $X right then but keep them informed on your thoughts.
If all is well there then I would get a survey along with mechanical checked (compression check, plug reads, drives checked, etc). I'm not sure what is included in a survey but get a full work up on it. Finding 1 thing more than pays for what it cost for a mechanic to go over it. If all checks out there then do the sea trial.
If the boat was represented as stated so far I would offer they seller $250-$400 "deposit" for a sea trail. If I buy it- comes of the price, if not theirs to re-winterize with. I know it's a buyers market but treating the seller as you would want to be treated if you were in each others position goes a long way IMHO. Shows you are serious about buying the boat since it cost them $0 for you to sea trial as well which the seller knowing you are a real buyer vs tire kicker may come back in your favor when it comes time to talk price...
Good luck and hope it all goes well for you!