Hi Mike, if I understand you correctly the car will start if you jump it, is that correct?
Ok, then a new battery should get you going. You do not need to install an OEM Mercedes battery but you do need to make sure that the battery you install is correct for your car. Here's a link to the Interstate Battery for your car. On your SRS light you will need to have the fault codes erased with a Mercedes scan tool. No doubt a low voltage code has been stored in the SRS module. If you give me the fault code for your check engine light I can help you decipher that as well. Let me know if you have any questions.
If I remember correctly the code was P175- (or perhaps P114 maybe confusing it with my wifes honda) something to do with the fuel rail?
Is it worth it to buy the OEM scan tool? I can do all of my own work!