First off, the person who put water in the radiator and left it that way in Illinois in winter, is an idiot.
Check the dipstick for a milky color. If it is, the motor is junk and the block is probably cracked.
Oil at the bottom of the shroud is collecting there from the turbulence in the engine bay and might be coming from the leaking valve covers or coolant mixing with crud that was there already.
Unless you volunteered to do the work, I wouldn't touch it. The best way to access the plugs is from underneath the van reaching up beside the engine which is risking your life without an assortment of jack stands and a good floor jack.
From all the problems you mentioned, I would suggest your boss buy's a new van instead of throwing money at a 100,000+ mi used engine.
There's no telling without a complete tear down what damage has been done and he's going to be wasting his money.
Also, oil floats on water, so the dipstick might not show the milky color. Other places to look are under the oil fill cap or, just drain the oil for an oil change.