A/C can be a crap shoot. We have cars that are pretty much in the neighborhood of 20 years old. R12 is expensive, but not impossible, however, we have known evaporator issues around 20 years as well as the expected rubber deterioration in the hoses and o-rings. I've done a lot of A/C work and live in the hottest climate, short of living in Saudi Arabia and a simple 134 conversion is about the best way. First see if you have any freon left in the system, just press down quickly and see if anything comes out, not that I condone letting out R12 in to the atmosphere, but a quick pssst to see if anything comes out is ok. If nothing, then you have a leak. The only way to see where the leak is, is to have pressure and dye in the system. So next you must flush the system, most places will do this cheaply or there are over the counter kits available, flush out the old system, and let it dry out. then you can have it recharged either professionally or yourself, either way, I would recommend that you have the system vacuumed, the little bit of air that is left will cause a pressure fluctuation and poor cooling. Recharge it with the proper amount of ester oil and a bit of dye. easter is commonly referred to as "conversion oil" because it is compatible with both R12 and 134, and is less likely to seep out you old rubber. charge with about 10-15% less that the R12 charge capacity, and enjoy it while it lasts, then hunt your leak.
The industry like to scare people in to thinking that most leaks are small, expensive and hard to trace. For the most part you can see leaks as oil seepage around fittings and where the rubber to metal seams are on hoses. if you cant see any dye leaks and it takes about a season to leak out, you have the dreaded evaporator problem. If you have a hose leak, they can be replaced either by getting new ones, or the way I prefer is I have a hose man that will replace the rubber portion of the hose and even repair the metal portion if need be, the can be the most inexpensive way because most low side hoses have a fuel cooler integrated in to them and are sky high!
134 in my 88 300E with the original compressor cools as good as I would expect, yes it does take about 2 minutes @ idle when its 120 degrees outside to get cool, but thats not bad considering thats pretty f-ing hot and my windows aren't tinted. Yes, by the book, you are supposed to change the drier and all the o-rings and the hoses may go bad as well as compressor seals. But find an experienced mechanic, who knows a/c well and they will tell you the same. Use 134 and trace your leaks from there. if you have to replace an o-ring or a hose because it leaks, it will be more expensive to put R12 in your car every time the A/C system has a leak, seeing as everything about is old! But if you do end up making repairs, the drier is cheap and if it helps you sleep better at night, throw one in. however the compressors are pretty tough, and mine has been working well for a while now, even under extreme conditions.