fuel - between 16-30 depending on how you drive
known problems - <2004 the valeo issue, late 2004 plus and you should be ok
- air shocks can leak
- supercharger clutch can start getting sticky and squeek
problems ive had
- supercharger pump needed replacing
- oil cooler leaked - replaced
- keyless handle stopped opening door - replaced
- passenger mirror wiring - whole assembly replaced
- TPMS - all sensors and control module replaced
as above many threads about them, go to mbworld.org, many many more posts on there too.
good luck, main thing is, check ALL the electric toys