You can use a flushing solution from the parts store or you can order it online. Typically there's a gun you load it into connected to an air compressor to blast it through the system then thoroughly blow it out.

O-rings are usable between either refrigerant. Get a set for your car and rock on.

R12 refrigerant must be used with PAO or mineral oil. You cannot use PAG. PAO and mineral oils will circulate with the refrigerant, PAG will find a low spot and pool, quickly ruining your compressor. Mineral oil was the original lubricant or R12 and is the preferred lube if you plan to stick with R12 forever. If you ever consider recharging with an HFC based refrigerant (R152a or R134a) use PAO oil since it is miscible with both CFC and HFC refrigerants (it also lubricates better than mineral oil).

When the system is charged and working, you should have a 30˚F split across the evaporator coil with the fan on its highest speed, all the doors open. Say if it's 90˚F ambient, you should be blowing roughly 60˚F air from the center vents if everything is right. Once the doors are closed and you're moving, the temperature in the cabin will pull down and you should be blowing mid 40s when moving, upper 40s to lower 50s when idling. Some degradation of the system may have taken place since the car was new and the big tube condensers used in the W123's aren't the most efficient, even with R12.

Window tint helps. A lot.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]