Well based on the codes you have issues on both banks of the motor. 303 is cyl 3 on the passenger side with 304/306 on the drivers side.

Some will tell you that removing or loosen the o2 sensors and run the motor and see if the codes pop up again...i don't see that as a safe way but I guess if one is careful then allowing some flow out of the openings may provide enough additional flow for things to run smooth. If you start with the rear sensors and all checks out ok then you can assume the plugged cat is the rear cats not the ones near the engine block. If you sill get the codes then you have to assume the blockage is in the upper cats and that they have melted.

There are some catalytic converter cleaners you can run through the fuel system which are about 30.00 and some claim they work, but if the cat is begining to melt down they will not do anything for you.

Now for the big news...exhaust parts on this car are over the top. Mercedes was the lest expensive with rebuit oem units with front and rear cats and pipes as 1300.00 per side...2600 for a full set from header connection to h-pipe connection. I chose to have a local muffler place here in town replace the cats only and that ran me 1000.00 for all four including labor.

If you don't have high milage on the car (80,000 or less) they will be covered under the factory emissions warranty by the dealer. Mine went at 130,000.