Starving for fuel is only one possible reason the engines are cutting
out around 2500 RPM.

Beware the "owner is having the fuel polished" excuse.
There will be no evidence visible to you, the buyer, once the fuel has
supposedly been "polished" unless you're down on the dock when the
process is taking place. As little as *nothing* could actually be done,
and how would you know?

If you're serious about the boat, pay *your* mechanic to analyze the
situation and express an opinion about both the general condition of
the engines as well as any specific problems noted. Don't buy the boat
until you now *why* the engines cut out at 2500, and it would be
typical to expect the seller to cure that to your satisfaction as a
condition of sale.

Asking the seller to cure the problem is often better than taking the
cash alternative. If somebody misdiagnoses the problem as "fouled
plugs" (and you settle for $200 off the price so you can deal with it
after closing) nobody but you owns that boat when the plugs are
replaced, the problem persists, and you learn that it's really going to
take another $2000 to solve the situation. Ouch.