1/2 a gallon a week (coming up!) is a lot - no matter what the source. and stuff like that don't get better on it's own, I am sure it started as a cup......my bet is that the owner is knowledgeable because the water ingress has increased over time and he has looked into the problem and fix, done the math and decided that financially makes no sense......and wants to sell with the problem declared so no comeback (makes sense), even at the cost of a low sale price
(low is better than none!).
The owner does seem clued up which is good, and also has sorted the stern gland in recent times should exclude that as source......I have no personnel knowledge of the boat in question, but whilst "they all do that" may be true (I dunno) - nonetheless most others boat makes / models do not.
Apart from the cost of repairs
(I suspect you are already thinking whether you need to do more than a quick bodge and then keep fingers crossed!), have to bear in mind that:-
a) keel may drop off at an inconvenient time (the bolts / glue / fibreglass will be deteriorating)
b) the water ingress may (will?!) increase - will need bilgepumps working 24/7. Most boats sink at the dock
c) a damp (or wet!) bilge will tend to smell (stuff grows in it!)
d) likely she will be uninsurable as is (you may be willing carry own risk, but depending on locale Marinas
may require it).
e) resale will be next to Nadda (and a slow PITA) or less than Nadda!
Overall, I would walk. or run! - even for free would be a long hard think (with a calculator!).
boats always have a cost, some can be handled (exactly what issues (and how many!) varies from buyer to buyer - mostly about what skills each have, but cash also important).....but some problems are best left to the Vendor!