Hello, and thank you for allowing me the opportunity to help you. If I am not online when you are, please allow me time to return. Are you looking at a 4 matic model, diesel, or just standard?
My reason for this is that they each have their own set of problems. The standard model is the most reliable one. There are not many issues with the base model. If you go with the 4-matic you will typically experience problems with the differentials and the cv axles. The diesel models are very efficient, but there are some issues with the oil injection system.
Sorry for not clarifying. The standard (base) model would be the rear wheel drive model with the coil spring suspension.