I drained the cooling system and radiator on my 1999 E430 last year. The block drain is on the right side of the block, above the motor mount. However, after I had drained the radiator I found nothing at the block drain. Apparently M-B designed the system so that everything drains to the radiator. So, I don't think you need to bother with the aux pump; it probably won't have anything to pump except for a bit in the heater core. It may not even do that since the pump isn't designed to lift, just circulate. And, there is the mono valve to deal with.