Have your monobelt tensioner checked. Also, see my recent post in the Hot Rods forum regarding a hydraulic hose.
I suggest running WaterWetter, BTW
As to a keyless entry/remote start system, I've had a Clifford unit in my 300TE W124 for a year now, no probs. It was about $225 installed, including the remote start (and MBZ requires an additional central lock/alarm interface).
I'll likely do the same thing, w/o remote start, to my 500. Cost will be $175 or so.
Enjoy your new car!
Best regards, Michael