Don't do the conversion. Find someone who does R12 and just recharge w/ R12. R134a does not "roll" w/R12 systems. In my 300E, I asked the mech about conversion and he tried to do it in the past on his own 300e and it started a leak. A leak taking about a day to fix that my own mech is too lazy to do it on his own 300e. Especially w/ the SL model, if you develop a leak expect thousands of dollars in repair.

In fact, the R12 system is a better cooling system.