I am a DIY and just finish replacing the evaporator myself with the help of the MB manuals. My recommendation is to buy the MB CD at this site, if you are to trouble shoot yourself.
I found the evaporator leak with the dye but, it took some driving to finally show up at the drainage hoses on each side of the transmission, the compressor front seal was also leaking (probably for working overtime with low freon). Since your compressor is not working I do not know if it will show up, may be somebody else can help you with that. I posted some of my experiences on this site, do a search. The evaporator (copper core is recommended)is relatively cheap, the labor is expensive and hard. I kept R-12 and it is working fine, so far.