How often do you check your oil? Basically, do you know the timeframe that the oil leaked to .5 quart low? My first thoughts were blown headgasket, but its weird for the oil to be coolant free still. I would try emptying the whole thing and putting some fresh coolant in, run the engine (idling) for a bit, and see if it changes color, maybe even just try distilled water...if the gasket's gone the coolant isn't that important (BMW vs just water).
If you have limited mechanical skills (maybe run a compression test if you know how) then you might be better off taking it to a shop to begin with and having them diagnose it. Like other said....warped head, sludge, etc. etc. can come from a blown head gasket being driven on.
The M52 is notorious for overheating and blowing the gasket though...