Error Codes
 

I have a 2002 ML320 getting the following error codes: P0304, P0171, P0174, P0300, P0304, P0303. I've already replaced the MAF, but the engine is still running rough when at idle. I'm guessing it's the O2 sensors, but not sure which ones.

The P0304 is often caused by clogged EGR ports.
There is a port for each cylinder....and they start clogging on the driver side of the engine....closest to the EGR valve. The last ports to clog are cylinders #1 and #4....farthest from the EGR valve..... With the other ports clogged....all the EGR gasses then go to the remaining 1 or 2 open ports.....causing those cylinders to not have enough oxygen.....and the misfire.
171 and 174 Lean Banks 1 & 2
300 - Random Cylinder missfire
then 304 #4 cylinder missfire
303 #3 cylinder missfire

I recently replaced the Brake Booster and the idle problem occured immediately afterward. Also, I hear a fairly loud hissing/whinning noise (like air escaping) from somewhere near the top-center of the engine.

I also recently replaced my brake booster and am having the exact same problem, minus the vacuum leak.