Both of those will end up being the om651 and are pretty much the same. E class has more interior room but you trade off on ground clearance and a bit less in the cargo dept. Hard part with glk I have found it getting a diesel that has options in it. Most of them I see have low line options with only I think 1 that has been just about loaded. I would recommend you at least get the good lighting.

One sensor per wheel. Remove wheel/tire and it will be held in place by an E10 bolt. Harness runs up to a junction connector behind the fender liner