These are generalities:
Lugger/John Deere-desirable engines-reliable, economical
Cummins VT-555 parts expensive and difficult to get
Cummins 6BT and 6CT-good engines, run well
Cummins Q series-newer, good-can run slow for economy w/o a problem
Caterpillar 3208 Good, reliable-better with less HP squeezed out
Cat 31xx series not highly desirable
Cat C Series-newer, similar thoughts to Cummins Q
Detroit x-53, x-71 are repairable, rebuildable, reliable but a little loud and a little messy. Better the less HP you squeeze out.
Detroit DDEC-avoid--less reliable parts availablity difficult
Detroit 4 stroke diesel--don't know but not many there
MAN-a lot in some Bayliners/Meridians--don't know
Volvo-some good engines, expensive to maintain, parts may be difficult
I forgot these:
Lehman-the 120 & 135-economical, reliable and parts available
Perkins-reliable, some better than others