R12 disappeared in 1994, parts to service it are long gone. Your only source for an R12 replacement is new-old-stock.

The TXV can't really be "tested". If the system wasn't open to the elements, the powerhead wasn't leaking, and the compressor didn't grenade and send shrapnel through the system, it's probably still fine. Install new O-rings after you flush the system and see how it runs.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

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