Wow, stoked for you.
Take tools, a battery, fuel, and a way to at least fill the fliat bowls to see if you can start it.
I would go and scrutinize every little thing that you need to repair, like does it run? Has it sucked up any rocks? Why has it been parked?
Everything today costs billions of dollars and it looks like the seller knows that operating that boat does as well, otherwise they wouldn't be throwing trash in it.
When you determine how much you want to spend, have that cash in one pocket. Fan out the cash in front of the seller. $3000 in bills laid out across the deck looks like a lot more than $4000 sounds.
In my experience, if I want something bad enough, I just get it and don't look back. How long would it take you to earn back the difference between what the seller wants and what you want to spend? Is it worth watching someone else driving around in your dream?
Someone bought what should have been my boat out from under me. I had already made the deal with the guy after a thorough inspection. Some POS walked in, with only 75% of our agreed price, right as we were starting the paperwork. The kid fanned out the cash and the seller changed his mind. I said, "I'm giving you full price." The seller said, "The kid's got cash now." That's never going to happen again.