Mercedes does not call the standard rear suspension "Airmatic". That is reserved for the manually adjustable height at all 4 wheels. This is a gas shock and spring system which keeps the car level according to load.

The gas bag / spring tends to go out around 100k miles as mine did. I think the best solution is to use Arnott replacements. They keep the original functionality and have a lifetime warranty. Because it is not a Mercedes factory part, your dealer probably won't use them. A local German car independent shop should be familiar with the job. I think mine cost under $400 each with labor included.

I can't help you on upgrading the front. I have had no problems in 145k miles and am happy with stock.

 

ditto
4 wheel airmatic was available for a while, even had ride height adjustment in cockpit!
Without airmatic, you get standard, rear self leveling suspension with the airbags. Yep all will need replacing at or before 100Km
One side will sag. Fix it soon or else your air pump will break from constant running.
Arnotts ar great, cheap, easy DIY. I changed both at once. Great phone support too.

The other thing that may go wrong are the level sensors attached to the rear suspension. If these fail you can change yourself but you have to go to dealer to get the ride height re-calibrated.

Sometimes the pump fails. but usually only when people have neglected to fix a slow leaking airbag.

Trying to retrofit 4 matic.. well, im sure it can be done.. but.... havent seen anyone do it in this forum.

 

Thank you all for the information and advice. Really appreciated.

I have replaced the rear air bags with Aerosus as well as the Valve Block (Original part). So far it works great.