The 6.2 GM diesil is pure junk,the Ford/International 6.9/7.3 are great engines and the DODGE/cummins units are good also.
Main thing to consider with a diesil is change the fuel filters often.The only cranking problems I have experienced were due to people using ether to start them,a tiny amount isn't bad,but most folks use enough to wash down the engine instead of cranking it.The other hard cranking condition is due to either fuel system leaks or worn/damaged injectors or pump.
In my experience rebuilding diesils the first thing you check is the compression,320psi is absolute minimum for a diesil,if you don't have at least that on all cylinders then heads,rings maybe the case.If it does have good compression then the fuel system is the place to start.
I have seen people rebuild their engines only to find out that the pump/injectors were the whole problem not the engine itself.
For my bet I would pickup a 3/4ton Ford with a bad/blown 6.9l and drop in a modern fuel injected 351Windsor,you will have plenty of power,cheap parts,good gas milage and long life.But by fuel injected I mean multi-port not throttle body,if your gonna do it go all the way.
I just need one more tool,just one!