Your system requires a can of A/C oil and two pounds of of R-12. R-12 is running anywhere from a $90-120/lb here in CA, plus the oil and service charge. You should figure about $250 or more to have your system evacuated and charged with R-12. You might want to call a couple of the local A/C shops to get an idea of prices in your area. When I converted mine to 134 back in '98 the local shops wanted $140 to just recharge my system, that is why I went with 134(only spend $70 on everything).

If you don't mind paying for the R-12, I would go with it since it is a well proven A/C system.