Buying a Magnum Core- Max Mileage?

http://magnummopar.com/articles-and-how-to/59l-magnum-engine-make-mine-a-59/

A link to a hotrod mag article which used a 5.9 with 160k on the whole engine, swapped EQ heads, a 380HP cam and made over 400 hp. The magnum engines use hypereutectic pistons and moly rings so they dont really have much cylinder wear, the 6.18" connecting rod also keeps wear down due to less side load than a shorter rod engine.( ie GM V8 5.7" rods)

I would pull the pan and if its clean inside, gasket, freeze plug and top end.

You will want to swap out the factory cam in all 5.9's as they are smaller than the 5.2 and keep the power low. At bare minimum, a 5.2 cam is a nice improvement. Dont waste a dollar on "fixing" the stock heads, EQ's are about as cheap as a valve job on stockers.