W210 4MATICs are quite reliable. Totally different from the system in the W124. Xfer cases don't leak--sometimes you'll have one that shudders on tight lock turns but that is often fixable by a fluid drain and refill with the latest oil and a bit of limited slip additive.

Fuel mileage difference really is negligible, so don't be too hung up over that.

Lastly, 4MATICS don't seem to be affected by spring perch rot, although they still merit inspection.

Good luck!

__________________
Regards,
Aaron