Just wondering if it is possible to fill up the A/C freon by DIY? and the car is S500 W140.

Other than requiring tools, training, and proper procedures, I see no reason why you can't DIY anything. There are only a few things more costly than crappy A/C work, though.

I am a pretty hard core diyer and my suggestion would be to find a good/reputable a/c shop that you can work with. Ask around and then go and talk to the owner/operator. I found a guy that is not only a good a/c person but also a good all around type of individual. I took the car to him and he told me what all needed replacing. He evacuated the system and I replaced the items that needed replacing and brought it back to him after which time he charged it again (r134 conversion 300D) and viola I have cold air :). On these hot summer days 60-65 dF blowing out of the vents when the outside air is 98 is good enough for me. If it gets too hot I drive the E320.

Remember this,

If you are adding refrigerant, you have a leak! If you add too much without the right amount of oil, you will damage the compressor.

Ditto to all of the above.

This summer I added r134 to my s420 '95 without any problems. Got a kit and some bottles of freon from Autozone. I's quite a simple procedure.