I have done to ML’s and one GL complete bags and compressor. I also tore apart one nonworking compressor. The “German” made compressors are built like toys… from my experience and reading they are good for about 100hrs.. I monitor mine and when they get close a unit goes with me on long vacations just in case. I have replaced the bags only and won’t do that anymore… had 1 Arnott that was very hard to seal on the front original bilstein. Bilsteins with ADS are going for about $450 a piece … that really is a no brainer just change the whole unit.
The best money you can spend if you plan on keeping the GL or getting another one is to buy a STAR system .. the Star checkout of the air suspension gives me piece of mind and functional/errors. I also go on the road with a MERCEDES OBD reader that has compressor on time reading ability.
So my advice first STAR system than if needed replace complete front struts with Bilsteins and brand name compressor and start watching the running time.. I check mine every oil change (66 hrs last check)!

 

That is both parts and labor. Compressor and valve block both replaced. The cost probably would have been less at a Indy mechanic but it was also in the dealership service dept for a new MAF which was replaced under the diesel emissions warranty.