I have 2000 C230 4 cylinder model with 50,000K miles. Just recently it has been starting hard. I have spent over $400 for new plugs, fuel system flush and filter. I have also spent over $500 for a new mass air flow sensor. It now starts fine after previously running but still starts a little hard when starting after it has been sitting for a while like overnight. The mass air flow sensor was the only code reading under the diagnostic which is why it was replaced. The technician's seem stumped. Any ideas?

who did and what is a fuel system flush.

has the fuel pressure been tested? like first thing in morning

How old is your battery? those suckers like to be changed ( or tested and serviced ) every four years

I would put a fuel pressure tester on the thing and let it sit over night. Most likely it is losing fuel pressure after sitting for an extended period of time. Maybe a bad o ring in the fuel pump/sender assembly. I've seen this on the newer c class cars, and have replaced both the left and the right fuel senders to resolve the problem.