the "fogs-on" thing that BMWS can do are not the DRL's. If you turn your light switch to markers only, you can hit the fog light switch and they'll be on. I donít know what if any purpose this serves but I always drove my 735 like that. My car didnít have real DRLs, so I kinda used them as such. Driving around dusk/early evening, car looks bad ass. My friend's '02 S4 canít do this, and my other friend's '96 Volvo T5R canít either. I guess most cars have relays such that you canít have fogs on unless your headlights are on. Go figure. A lot of American cars I believe can have fogs-only on.
If you get 500E headlights, I'd opt for putting xenon in the headlights and since you loose regular highbeams, up you should rewire your highbeams to the driving lights so when you use the stalk to work the highbeams, the driving lights turn on. Reroute the wires such that the fog position on the switch will operate the new fog lights.