Actually, under the EPA rules (I hold an EPA HVAC license here in the US) there wasn't much to consider design wise.
I utilized all standard R-22 AC components, but instead of utilizing R-22, I purchased a "mix" (http://www.es-refrigerants.com/ ; no affiliation, just a customer for more than a few years) and installed according to directions. This was done after an electrical brownout burned up my old system in late 2008.
Replacement system was identical in size and efficiency rating, but I'm seeing lower electric bills as a result.
The "best bet" (not advertised) is to do the math correctly for charge amount, and to have a TXV (thermal expansion valve) on the system (standard on many newer systems) to regulate flow into the evaporator (cool side coils). My system charge was less than 4lbs of coolant.
I always get the safety question, but there's a natural gas line going into the furnace already with a lot more supply than the charge in the AC system!
http://www.refrigerantsnaturally.com/home.htm is a good location for info, and members include mcdonald's, cocacola, the UN environment program, etc.