I have seen the Cats come loose and knock a bit. I always start on the easy side then work toward the hard stuff.
On the A/C, I have not been able to get a good 134 conversion on a 300E as of yet. The system was not set up for the larger molecules of 134 and the oil gets absorbed by the hoses over time.
I would find the leak, then use 12.
PS, you can check for evap leaks by taking out the blower motor and looking in.
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS