If your current system is totally stock and it has schrader valves on the A/C lines then it is still R12. If it has been retrofitted it'll have a different style valve screwed onto the schrader valve, like an airhose fitting.
Make sure the compressor is kicking on first of all. If it is, move to the recharge kits.
If your R12 system is blowing barely cold then you have some R12 in it still. You can get some stuff called "freeze12" or have the local shop fill it with this stuff and see if that's all it needs.
If it doesn't blow cool then you may not have any R12 in it at all. Mine was empty. You can choose to go to a shop and vaccuum out the system before filling with R134a or chance that the system is empty.
If you want to spend as little as possible then you can go to walmart and get an A/C recharge/retrofit kit for under $40. It has fittings you put on the shcrader valves and three cans of R134a. You just screw on the fittings, fill it with the R134a and it should workout ok.
If you have a 318is '91 then the system has a newer compressor with seals that work with R134a from all I've seen.
My personal experience is this:
My system didn't work since I bought it, and the PO said it didn't work. I had no evidence of retrofit. I figured I'd try the cheapest method before paying $400 or more to do the whole thing. I got the walmart kit for $36, screwed on teh low side adaptor and put in one can of R134a. The system now works and has worked for about 1-2 months with no issues. It sure is nice having A/C!
No, I did not vaccuum the system, I did not purge the system into the air, I did not replace the condenser, I did not change any seals.
I didn't do anything but put in one can of R134a.
It works fine so far and if it craps out I am more than willing to pay the $400-$600 to repalce th whole deal, but I'm happy with $35 for now!
I would say that the stock compressor puts a good load on the engine, I can see it on the mpg meter and feel it. From all accounts the newer compressors don't put nearly the load on the motor. I'm willing to change that out in the future.