Now you're getting into an area where there are different ideas and opinions. I personally will not contaminate a 12 system with 134 traces or vice versa. There are others who are now saying that there is no problem from that.

A 134 set up is more expensive than an R12 setup and there are plenty of R12 setups available cheap at garage sales, ebay, etc. I would recommend getting a decent 134 set and find a 12 set on the cheap somewhere and never the two shall meet.

I also would recommend your staying with 12 on a system that is originally 12. I have even reverse converted back to 12. You can take an online, open book test for $15 and a few hours spent. Then you can buy 12 on ebay for about $15 per pound. You could buy a 30 pound cannister for $300 or $400 and have enough R12 for years.

Good luck,