Don't take this the wrong way, but you're asking some pretty basic questions about your a/c system.
I would suggest having the work done for you.
You could really do some major damage to your system if you're not sure what to do.
In particular, you wouldn't want to harm the evaporator, which would drain your bank account.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".