Wish I jumped on this thread sooner.... but
I see NO reason for any sea trial or such until the seller has accepted an offer subject to. Also, you can include that any undisclosed issues he pay for and get that agreed on, too.
As for a great boat with few issues, absolutely you can expect that with a 40 year old boat.... IF it was maintained properly. ALL boats have issues and maintenance, but clearly, this one has been neglected, and you'll have to deal with that.
I could make an argument that ANY major item that is an undisclosed issue be complete prior to purchase on the seller's dime, and items that are disclosed prior to offer you need a concrete estimate of repairs just to know where you stand.
Now, sounds like there's been a few improvements which is great.
When you figure out what this boat will cost you in total take ALL of the discrepancies and all your wants and add them up. You well end up well north of $100K, and probably will, but that may be doable and end up with a really nice boat.
Another thing is time. It will take time to fix, modify and make additions and stuff you want done, and that has a value. From the sounds of it, I'd bet that you'll tie up 6 to 12 months or longer before it's done the way you want. But depending on the projects, you may have use of it for most of that time. If you get into major engine work, replacing fuel cells (which I'd factor in because of their age), and major restoration, like the blisters, thru hulls, etc., you won't have use of the boat during those repairs.
This is clearly a project boat, but perhaps not a major project. And if you like to fix things, modify things and do a lot of maintenance work, it could be fine for you. It's not my thing, however.
When I bought my boat, it was 20 years newer, with less than 500 hours and pristine condition and took me over 8 months getting it the way I wanted it. Just food for thought.