I had the same problems with my 300E when I bought it.
Mercedesshop had a rebuilt compressor/clutch for around $500, which is what I went for. But it may be as simple as seals - one on the speed sensor (back of compressor) and two where the compressor bolts to the manifold pipe. To see these with dye, you really need to take the undershield off and look from beneath.
I went ahead and did the R134 conversion which means you need two switches and a new expansion valve plus a drier bottle- this is a hot topic and there is a ton of posts on it.
I bought the sensors from Mercedes along with shocks for about $500. You need to upgrade the cables and the wear sensors and some other stuff if it has not been done. BUT - getting the sensors working led me to the next problem which was a an intermittent loss of power to the ABS computer.
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe