In my hurry to reply before bed, I forgot to explain some of the bigger deciding factors of choosing r-134a. Using a constant of 54* evaporating, 150* condensing temperatures.
R-410a pressures would have been pretty extreme, 155/610psi. R-22 a bit more acceptable 90/370, but not oil compatible. R-290 was considered for some time, given 85/327, but seeing as r-134a was 51/260 which are much more manageable, and available in bulk right out of the bottle. It also works with the existing POE, and mixing a little PAG in wont hurt it as well (using automotive charging equip that has PAG in it, as well as the use of dyed oil for leak finding). I didnt bother considering R-12, since its becoming so expensive. Something you may get a kick out of, is look up the blends in Freeze-12 (a "dropin" replacement that we used. I feel very ripped off). And speaking of which, AC-Hacker, you will like this link. The epa did something useful!
Composition of Refrigerant Blends | Alternatives / SNAP | US EPA And to all you r-290 supporters, you may be entertained knowing I dissected a hotel minifridge to discover the compressor was rated for r-290, r-134a, and blends (USA Model)