Before I agreed to a new compressor, I would check to see exactly what is leaking. You should be able to tell from the dye. There are seals between the compressor and the manifold pipe, and on the speed sensor on the back of the compressor. You can get them from the dealer for a few bucks and replace them yourself if that is the problem.
If you convert to R134, you need to replace the Schraeder (expansion) valve and two sensors which come to about $100.
You really need to take it to a shop to have the system evacuated and charged.
R134 conversion is a hotly contested topic on this board. In fact the whole area of 124 climate control has generated an amazing number of posts, which you make want to look at. Including my epic struggle with a fried compressor. Don't miss Steve's classic on Black Death.
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