The concern with 'new' is getting the appropriate power plant...new big blocks are hard to find...Mercruiser used to offer rebuilt by the factory options. Dummy-me didn't fully appreciate the 40' aspect in the earlier post. Personally, a pair of 502's would be preferred for gasoline - diesels would be better with that much mass...Most of the 'new' OEM options will use the GM small block and I don't think they produce enough torque if you wanna run up on plane...they can be geared down but they will be working very hard...
'new' also infers the catalyst exhaust system...and thats a different discussion...when they need changing, it isn't cheap.
if 'well-maintained' doesn't have a log book with a stack of receipts, its just a 'marketing phrase'...
finally, on the cost side, $15K would have bought a bobtail engine a couple years ago...you can find turnkey rebuilt big blocks with about $17K-$18K....and add a few thousand more, each, if you want new gears.