Diesel in the oil is bad news. It indicates leaking injector(s) that wash down the oil off the cylinder wall(s) making for dry contact between the piston rings and cylinder on start up and excessive wear that leads to more blow by, really dirty, contaminated oil, even more blow by and eventually lowering compression more and more on that cylinder. Not to mention the damage that can/will be done to the rod, main and cam bearings and other wear surfaces like cam lobes, lifters/bores and timing set, too from diesel contamination.
But that is my worst case scenario. My "best educated guess" is that the overfill condition it due to the direct actions of the PO - the beastie was 'using oil' at an accelerated rate for whatever reason (leaks/burning) and it was overfilled to prevent running too low/out, below the ADD/or off the dipstick - to prevent catastrophic engine failure. Just MHO.
But yes, get it back down to normal 'Full' level and do a quick 'nose test' for diesel in the oil at the least, before proceeding further toward getting it started.
Remember, you need three things to get a diesel to start/run: Fuel, air and enough compression to heat the air hot enough to ignite the fuel (with or w/o the glow plug system while running, with the GP system to start from cold).
It may be time to think about an individual cylinder compression and leakdown test to see if lack of compression is the culprit for the lack of start/run for this beastie after verifying that you are indeed getting fuel to/out of each injector and that there is not a mouse nest anywhere in the intake from airbox to lower intake manifold (LOL) blocking airflow.
