I hear you! If yudo a search, you will probably find a lot written on this BB concerning AC and the pros and cons of R134. Larry Bible from Texas provides a very good summary in my opinion.
Some things to consider, R134 is cheaper, but is not as efficient. Adding it to a system designrd (sized) for R12, will have a performance penalty, but you will only begin to notice it when your AC is running truly raunnib at max capacity. If you need an ice cold car on the hottest of days, you may not be satified.
R134 is not compatible with the seals and oil used with R12, so a complete rework is required. Do not just dump it in, that doctor, I mean mechanic, was a quack!