I agree with Sailhog here. You have identified a brand of boat that appeals to you. You want to make an offer but are not sure what the boat is worth - to you or to the owner.
Start by finding out how many of these boats are on the market, and what they are selling for. Is your boat in that range ? If not why not ? Next you need to determine what level of skill the person who "finished" the boat had. Is this a case of someone throwing in some pieces of residential grade marine plywood and painting it prety or is there beautiful joinery ? Pictures can tell you some of this, but the best thing you can do is to get a surveyor to talk to the person who fit the interior. They can tell a lot by asking a few questions.
If you are buying the boat to sail offshore, the interior needs to have been constructed in a way that will allow you easy access to to the hull, in order to repair, replace and install things. If it wasn't, then you have the choice of going to sea and hoping nothing ever goes wrong (highly unlikely) or ripping lots of things out and starting over.
It would be a very good idea to do some serious research on the net reading articles about and by people who are actually cruising offshore. What boats are they sailing ? If the brand that you are looking at is not mentioned occasionally, then this may be an indicator that it is not a good offshore boat, and vice-versa.
Remember that if you are buying a home-built or a kit boat, the market value of those boats is considerably less than factory-finished units. It will be when you buy it and it will be when you sell it. So are you going to hold on to the boat for a long time, or are you planning on selling it in five years ? If it's the latter, then think long and hard about how much money you are going to sink into it because you will not be able to recoup all of it. A general rule of thumb is that if you buy a boat in today's market, and maintain it nicely, upgrading things that need to be taken care of, you should get back your initial purchase price - but likely not your purchase price plus improvements.
You were asking specifically about purchase strategy...well you are here and the boat is there. The thing you need to find is a neutral third party to assess the boat. I would suggest finding out who the most reputable boat yard in the area is, and asking them to recommend a surveyor. Call the surveyor and arrange for him to interview the owner/finisher of the boat, inspect it and then give you a report on his findings.
It is going to cost you money, regardless of what you do. Boats are like that.
Good luck and please update here with your progress.