Thanks, I too have done R-134 conversions and have worked on A/C systems since the late '80s. Along with using Ester Oil as your conversion oil for the system, the usual suspects are a slight drop or lack of cooling out of the condensor if it's sized or tubed for R-12 when you convert the system over to R-134. Evaps, valves, lines are usually not an issue. O-rings can always be changed over to R-134 material if needed. So, my usual biggest concern is whether the compressor can handle the higher pressures that R-134 puts on the system, and if there's shortened service life from that since compressors aren't usually cheap, even with a rebuilt.
Getting R-12 isn't an issue either, aside from gulping at whatever the wholesalers are charging for a 30 lb can each year <img src="/forums/images/graemlins/smile.gif" alt="" /> Yeah, R-134 at Pep Boys retail, or from your jobber in a 30 pounder is much cheaper, so I'd like to go with that if possible. <img src="/forums/images/graemlins/patriot.gif" alt="" />