Personally, I stick with R12 in all my cars (that were originally R12) because I can. R12 has a slightly cooler boiling point and a relatively cooler melting point. It's nice to know someone in the Auto AC business! If you do decide to do the conversion, be sure to completely drain the compressor of oil and use R134a oil - if you skip this step, you will not experience the arctic air you are expecting.
What your car came with from the factory depends on the year. R12 was used in vehicles up to 1993.
How much oz of refrigerant is needed to fill up the system? Occasionally I still see some R12 cans offered and just wondered how many I would need. Environmentally I figured since these cans (that are usually old and still sitting in someones garage) are in existance already anyway, and will erode away and expose their contents, I might as well still use them.
If you can find the R12 in 12oz cans that would be fantastic. A complete recharge is 650 grams or almost three 12oz cans. And - as long as the metal seal was not broken the R12 should be fine. So, your best bet may be to find those cans!