Not sure on W123, but on W126 the only options of subframe bushings are Meyle. $80 for a set vs >$300 for MB. For the ENORMOUS difference in price, I can change the Meyle's nearly 4 times, so that's the route I went. Having done it on 2 W126's so far, they seem to hold up fine.

On the 126, the easiest way to change the subframe bushings is to jack the car up and put it on jackstands. Drop the bushing retainer bolts and the center through bolt. Jack up the subframe and insert a 24mm socket on top of the subframe mount. Lower the jack and let the subframe push its own mount out. Lube the new bushings with some silicone spray and jack it up into the hole with the jack, use the old bushing as a pusher. Reassembly is the reverse of removal as they say.

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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!)