Given the facts that we are currently experiencing one of the best buyers' markets ever and that there is a surplus of good used boats available, why would you take a risk and buy a boat in reliance on a promise that a repair would be made?
If I read your post correctly, you have not yet physically inspected the boat.
Google "Seafarer sailboats" and discover the facts. Don Casey and many others suggest you buy a known quality boat, that means a boat that was produced in great enough numbers to establish its value, its quality and its likely problem areas given its age.
Well Seafarer sailboats are a fairly well known boat, while not the hugest volume producer (not really an indicator of quality) they are known to be fairly high quality boats, nothing that would scare me away from a good example.
I would consider the repair estimate very low, assuming you want a quality repair. Refinishing a repaired deck and cockpit is far more difficult and expensive than the hull - you are looking at the deck and cockpit, up close, all the time you are using the boat.
I would expect a proper repair, proper being one that is not immediately obvious every time you look a the deck, would cost at least three times the quoted estimate.
The boat is not worth anything more than the salvage value of the engine, maybe not that.
Yes, this is harsh but likely true. With so many good boats out there the only way this would make sense would be if you offered the guy a $1,000 and have it trucked to your back yard, and do the work yourself. I don't think any contractor is going to do a very good job for $5,000. I would think you will want to paint the whole boat, as it will need it anyway.
I replaced about 12 square feet of balsa cored cockpit sole this Spring. It took about 25 hours but wasn't very complicated -- I coated the sole with Kiwi Grip and made no attempt to replicate the original non-skid. At $80 per hour the labor would have been $2K.
None-the-less, I would suspect that the current owner would be better off scrapping the boat rather than investing the $5,200 to repair the deck. The combined value of 4,800 lbs of lead ballast and the diesel exceeds the $8,300 the owner would get net of the cost of deck repairs and sales commission.
I will say to that I looked at a Seafarer 36 and I stepped on the deck and bounced and it was like a trampoline. The rest of the boat was a mess, and I told the broker I would not give the guy a $100 and he told me I was right. He only listed the boat because the father of the owner was a friend of his.
I am still looking. Lots of boats out there in my price range, just not a lot of good boats.