Thanks for the feedback guys, it definitely helps. I already assumed what you both said re: the compression test...wanted to hear it from somewhere other than my gut / research.
CJD, as far as where the noise is, I definitely don't think it has to do with the v-drive / tranny or prop coupling, as it was happening out of gear as well. It was definitely coming from the engine and was definitely synced in timing with the RPMs, so I imagine it was something happening internally. Hard to say if it was coming from the top end or bottom end in there.
I think your suggestions on a warranty for just the engine or bottom end are awesome suggestions, and I hadn't thought of them previously so thanks for the help. I think the boat is a great deal otherwise -- I am getting ~$37K from NADA as far as value, and I am seeing similar boats out there listed in the $30-$38K range, I'm hoping to walk away with it for WELL under $30K (we have agreed contingent upon what their mechanic diagnosis). It is in practically show room condition, other than the wiring between the amp and tower speakers needing some work, and the depth sounder read out screen seems to be shot.
I will try to get the boat running again and see if I can't more closely identify where (specifically) the engine noise is.