If you plan on getting 500K and don't mind getting dirty, you might as well start working on it. The engine may last that long, but the transmission will fail and you will have lots of leaks and other minor issues. Just google most of your problems and you should be able to fix most things. The turbo pedestal bracket is known for leaking, as is the high pressure oil pump(hpop), so figure out if the leak coming from the front of the engine or the rear. Don't let the fact that it's diesel scare you. There is a bit of a learning curve, but 7.3s are simple to work on compared to most of the newer diesels.