Go ahead and do the conversion. If you have lived for some time with no AC, then the ~90% (vs R12) cooling you will get from 134 wiil seem like a godsend. Does your car currently have refrigerant in it? If so, you will have to have it removed and captured by a certified tech as you can't release refrigerant into the atmosphere. If there is no refrigerant left in the system you will want to pull a vacuum on the system to make sure you don't have any major leaks. You can do this yourself with a cheap set of AC gauges and a vacuum pump from Harbor Freight.
If you have small leaks, you might be able to get away with a sealer like Cliplight Super Seal Pro ( this stuff seals leaks in metal components as well as rubber and I am told it is used by MBUSA as an alternative to replacing evaporators). You will also probably want to replace the drier or at least heat the old one while it is under vacuum to remove any water that has accumulated in the dessicant over the years. I have converted several cars over the years, mostly American, but also an Audi and one Merc and I was not able to tell much of a difference between R12 and R134.
Freeze 12 is a mix of R134a and R142 (propane). One should not top off R12 with Freeze 12, i.e., mixing R12 with R134a. People do it anyway because it works short term.
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