What boat is it? Different factors E26 vs. E38. If it's a listing, post it and the group will have comments.
I ask because, re the spring launch, I would think the owner will have to do that. It's easier to sell a (big) boat that's in the water.
The "sea trial" is mostly about the sails, the rigging, the engine. Good to see everything in place, makes inspection easier. The sailing qualities are the least of it, because model to model they're predictable.
On the other hand, the best inspection of spars and rigging is when they're down, and a surveyor can do most of his other inspections on the hard, too. So if you;re interested, take advantage of the opportunities.
I don't think it's crazy to survey the boat now, lock in a refundable deposit, and make the contract final after a sea trial in spring. That is, if you're ready to commit and have not been able to find another boat suitable.
You'd still be able to back out for no reason at all. But the owner would know you're serious, and have spent the survey money, and are the "real thing." Boats are hard to sell.
Good luck. I myself would probably have this boat burning a hole in my pocket all winter, worried somebody else would pip me at the post (hahahaha--they won't), trapped in double and triple-think, unable to stop planning improvements, and wondering if spring would ever come.
You may be more stable. Let us know how it goes.