If the fan is engaged even when you're at highway speed and the coolant temp is below 100C, that's not right. Mine disengages as soon as the temp drops, 10 seconds after the fan comes on, tops. Go with the OEM clutch. I prefer to go with oem everything. To me, if mercedes considers it good enough, thats all I need to know
. Also, when you're sitting in traffic, it's not uncommon for the m103's to get toasty. Mine does, especially with the a/c on. The electric fan is supposed to come on when the temp reaches 105C. Just so long as it cools right off when you start moving again, you should be ok. When you take the the clutch off, inspect it. If there's something wrong with it, you may be able to get your money back. $400 for the pump at the dealer ain't too bad, especially if you're gonna do it yourself. I would be a little leery of having to make spacers and stuff in a M-B water pump. But, do the fan clutch first, before you spend the money on the pump.