Spose your lucky your not in australia, as its 100% illegal to deal with R12 now, all R12 cars must be retrofitted to R134a.
I do these all the time (I'm an auto electrician by trade and also have a my Air-con license). I've retrofitted many cars and in nearly all of the cars theres no difference in the cooling capacity. during summer we obviously do alot of aircon (honestly, about 2 weeks before summer, than all through summer till about a month after, most of our jobs are air-con related, its not uncommon to come in and see the day book with twice as many air-con jobs as any other job) and even on the hot day a properly retrofitted R12 system can easily get down to a frosty 4-5 Degree's (celcius...) (we usually test at the center vents, all vents open, fan on a medium to low setting (for example, on a setting of 2 in a 4 speed system), and flaps on recirc).
Yes the R134a will leech out of rubber piping faster than R12 hence why majority of R134a designed cars have solid tubing with the only rubber sections being from the chassis to the compressor its self, saying you'll be refilling daily is completely incorrect, if any system is releasing gas on a daily basis i'd be checking for leaks and also checking your pressures ! (alot of cars have compressors with relief valve instead of over-pressure switches, which inturn when the high-side pressure gets too high, just dumps gas rather than turning off). That said, yes, it will leech out quick, but you'll be back every few years not every day (tbh, most of the retrofitted cars i've done i havn't seen back for years and often only come back because the the "next service due" date sez so, not because the "air-con isn't cold".
I dunno what the A/C refrigerant lawa are like in other countries, but over here R12 is outlawed and i'd hazard a guess and say the laws would be somewhat similar in other elsewhere also (more-so because of all the global warming crap and so on, i know our A/C laws, even though R12's been outlawed for a while, the actual handling of gas and how A/C systems are serviced and general rules and regs have gotten very strict in the last year because of it).
so yeah, just get the car retrofitted, rather than trying to source R12 (whether legal or not in your country) or going down some exotic gas route. I've never had any problems with retrofitting and as i said, i've done literally hundreds of conversions in my time
ps, Re: changing oils, most people will just use a Polyester oil, as its compatible with the mineral oil of R12 and the PAG oil fo OEM R134a systems plus its much less hygroscopic (ie, doesn't absorb moisture nearly as much as PAG or Mineral oils do). As such, removing every last drop of mineral oil isn't as much of a concen as the Ester oil will just mix with it/absorb any left over mineral oil after evacuating the system and changing the reciever drier and eventually just deposit the mineral oil in an out of the way place, like the drier or accumulator, which will mean next time you get a service the any oil in these places will get removed when you replace these parts anyway
I suggest you just google airconditioning retro-fitting and airconditioning ester oil and such, theres plenty of good sources out there that'll fill you in as to what exactly has to be changed in a retrofit, what has to be added/removed and what the deal with tubing/o-rings/oils/etc are