when someone who is selling a boat says "it just needs a little valve job", you should expect that there will be plenty more related expenses that will be incurred because of it...
a properly maintained engine has very little valve issues over the life of the engine.... an valve adjustment every 2000hrs is normal, but when more is needed than that, it probably wont stop at the valve train....
I have seen valve seals fail prematurely, but its rare. so its hard to give any optimistic advice, other than saying a simple valve job is a much better option than an engine swap, if in fact it really is just a valve problem.....
work out a deal on the boat with the owner to share the cost of the repair, (it can be written up in the legal paperwork of the sale so your both protected) so then whatever it comes to, you wont be stuck with the complete cost of a re-power... he wants to sell and you want to buy, but im sure neither of you wants to take the entire risk of what is going to be found when the engine is opened up... if he is not open to dealing this way, then he is opening the door for you to lowball him without any shame in your offer.