I agree that the chain should be checked. This is a relatively simple thing to do.
As for a $600 R134 conversion kit. I can't imagine what's in this kit. I sure hope that there is a compressor in that kit for that much money. These systems are changed to R134 in many different ways. Some folks just pump them down and shoot them full of R134, I wouldn't suggest that method. But I think a $600 kit is way beyond the opposite end of the changeover spectrum.
If you can find the leak and recharge the system with R12 it's probably best. However, if you didn't indeed repair the leak, you'll lose the "gaseous gold" to the atmosphere. It will definitely hurt your wallet more than the atmosphere. A more practical changeover than the $600 kit would probably be to empty as much oil from the compressor as possible, replace with ester oil, replace the filter drier with an R134 compatible one, and recharge. If you want to go a step further, also replace the o-rings everywhere with the R134 type o-rings and flush the system.
The pros on this site have converted many of these, they may have some better suggestions.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in