I had QSC 500s in my Grand Banks and they were completely trouble free. You need to look out for a couple of things. First, find out if the aftercoolers have been serviced. If not, knock off a few boat bucks from the purchase price and put that on your list for immediately after you buy the boat. When you get the after coolers services, make sure it's by someone who knows what they are doing. They need come completely apart, be cleaned, new o-rings, greased like crazy so they will come part the next time, pressure tested, and reinstalled. If you want you can sent them to Tony Athens for service. Remove and reinstall isn't too hard.
The other thing to check carefully is to see if the boat is propped correctly and whether the engines have been overloaded. That will kill a good engine very fast. Look up the rated RPM for that exact model QSC, then sea trial the boat at wide open throttle. The engines should EXCEED rated RPM by 50 RPM or more. This should be possible with full fuel, full water, everything on board. If they can't get over rated RPM, then they are overloaded.