Part Number is 220 320 25 38, price was around $62.00 to $80.00 for the rebuild kit from; (www.oempartsdirect.com) they are fairly complicated to repair - you will need the tool to remove the circlip on top of the strut. The second tool is to remove the brass valve and insert a new one without scratching.
Otherwise, it may be just as cost-effective to replace the assembly with a new unit. Try the many on-line auto parts sources, such as; www.RockAuto.com, www.autopartswarehouse.com, or www.partstrain.com for the replacement parts.
You may order the kit on line from www.aboveallmotorwerks.com, the part # is 220-320-25-38 the same part number if you try to order through the dealership, just about $30.00 cheaper in some cases at $115.00 apiece vs the dealers $135.00 - 179.00, depending on the dealership. You can also purchase the tool on ebay from a company called kinetik (auto equiment & tools inc), again the price on this was $85.00. It is called a Mercedes Seal Cap Remover Top Air Suspension Strut Tool. As far as the job it is pretty straight forward.
(Note: This guidance is for general information and is not intended to substitute for informed professional advice, use at your own risk.)
Take and remove the air lines. (car supported, stands or lift, weight off the suspension.) Then tape the air lines ends up, cap the nipple with tape as well. (on the strut cap.)
Now, take a small chisel after the resin in the old cap, ideally it will come out in pretty clean solid pieces. Once you have all that cleaned out you take and remove the old nipple. Next you attach the strut tool to the strut tower to press the old caps down far enough to release retaining clip. The cap has to go down roughly 5mm to get the retaining ring out. Once the clip is out the cap will basically spring out on its own.
Clean the hole up, lube the new cap and use your tool to press the new cap down enough to set the new ring. There is a recess in the new cap that the ring has to lock into as well as the groove in the strut cap. You'll get a better visual once you have the old one out.
Once the cap is seated install the new nipple and airline and you are ready to go.
Before lowering the car you need to start the engine to allow the pump to repressurize the strut prior to letting the car all the way down.
The press is pretty simple and lightweight. 3 legs set to match the strut tower studs. Roughly 6" tall. Then the center is a big washer that the legs are welded to and then a 12mm nut is welded in the middle. When setting the tool you will see that it fits over the three strut tower bolts. You need to attach the tool one nut at a time to prevent strut from slipping out of place, again this will make more since when you get into the job.