They are really good cars.
I'd stay away from the ones with turbos but that is just me. In my opinion get the one with the M272 engine. Personally I think it is a more reliable engine than the M276 BUT don't go on just that because I'm at 313k miles on the GLK with the M276 engine. The M272 has a double row timing chain
. The M276 timing components will require a little more maintenance over 300k miles.
Don't over look the GLK too. I like the wagons a lot but I believe I like the GLK better just because it is easier to get in and out of.