ST185 owner here who JUST got a long dead A/C system fixed.
Depending on where you are, an r134a conversion is going to be either recommended or necessary. The fitting conversion fro what I understand isn't hard, and my local shop did it for me for about $100 all told. However since your system isn't working properly you'll have other things to fix too, and it can quickly cascade into a much larger project in hunting down and fixing leaks after the conversion. To do it 100% proper you should really replace *all* of the O-ring seals in the system with r134 seals, although some shops will offer to try and do the conversion without (as mine did). In my case after doing the r134 conversion and replacing a couple seals and a hose, they found a slow leak on the compressor itself, which is not an obtainable part. It was patched with silicon gasket maker and seems to be holding a charge now.
Good luck.