the solvent flush is to remove the oil necessary for R12. the vacuum pulled is to boil out water, and remove all the air. the conversion kits use conversion ester oil, which isnt fantastic, but works.
it gets 100 F in summer here, and im very comfortable in my R134a mr2. my other mr2 i had to kick the fan down a notch because it was too damn cold after a while! i think mine has an excess oil charge or it would be freezing cold too.
my R134 will freeze your balls off.. why pump flammable, dangerous stuff into your car? not to mention no one will service or touch a car with substitute refrigerant in it.
its been 7 years on my conversion, and its running great
how many times do you mind bursting into flames? i prefer zero.