It shouldn't take more than two tries to start the cold engine, cold engines will need choke but warm/hot engines will need no choke. Stop warm engines and wait 10 minutes, then they should still restart right away very easily.
NO HESITATION during acceleration, no quitting or dieing out, no nursing throttles to keep engine running.
And, don't forget, you need to check the compression and confirm it's very close to 150Psi(787/720 motor), 120 Psi is an engine in need of immediate rebuild and really anything less than 150 Psi is suspected rebuild, it's indicative of a damaged piston. Seems like they either blow ~150 or they blow 120 and less.
For $8k, you can expect a boat this boat should be in great cosmetic condition, the vinyl seat covers should not need replacing and all hardware should be tight and in place, no missing parts or ugly cracks in the gel coat.
Check that all gauges work properly and the controls work smoothly, the steering motion while on the trailer should be smooth and easy, the forward/reverse shifter and throttles should not bind. While underway, steering will require more effort than if the engines weren't running, and some pull to the right is normal and expected.
See if the fuel lines around the carburetors are gray colored, if yes then they need to be replaced b/c the gray ones are carburetor cloggers.
Look the wiring over, and the mounting of the stereo, sometimes people are sloppy when running cabling for stereo systems and hacking up the original harness is a no no.