How much is an equivalent UK suplied boat?

I would be looking to reduce the asking price to cover all your concerns, VAT certificate can be obtained from HMRC, get the currrent owner to pay the VAT then hand you the certificate or reduce the asking price by the same amount & pay it yourself.

Factor in the CE marking & engine & drive service then get a surveyor to check her over.

It may have stood for a while if the current owner didn't do these things prior to purchase & wont take the hit now he's trying to sell.

Dont forget as a buyer you have the upper hand here, if you cant negotiate a deal then you can simply walk away.