I'll have a go, I'm new to mercedes but have a fair bit of knowledge on other cars.
If it smells rich then I would check your fuel pressure, if the regulator isn't working properly it could be supplying to much pressure to your injectors, causing it to run rich.
Or you could have injectors that are just stuck open, unlikely though.
You need to do a compression check first. Then go from there. If the comps are good then you can start to do some fault finding.
good luck,
Grant.