I have same problem with my 95 E420. they found 3 leaks--expansion valve, a hose and evaporator. First, they recharged system to see how long it might take to deplete. Well, it didn;t take long about 3 weeks. So, what we did was go in and make "easier" fixes first. They replaced the hose, new expansion valve and new receiver/drier....bill was about $500...better than the $2000-2500 for also doing the evaporator. This was done about 5 weeks ago and air is still very cold. Although, in recent couple of weeks the weather has not been as hot, like 90's as it was.
I cross my fingers, this will take me through many more months as I do not want to face the dreaded evaporator fix. I would try the simpler fixes first to see what happens.
Of course now with cooler weather coming on I can do the 460 air conditioning...60 MPH with 4 windows open
95 E420 ( 76K) SOLD