imho oe cats last a long time.they sometimes break because of very rich conditions or impact.
but the codes you got were not for running rich.
if you ever have to replace 02's again, just make sure that it is just the ONE 02 that is bad. two or more at one time points to a vacuum leak or mixture problem. 02's can last 200k miles. just may be a little slow.
mercedes are ment to be driven at high speed. that should not damage the cats.
a bad mechanic is more of a probem
when i see more than one 02 code on two differant banks,the odds of bad 02's are pretty slim
some thing i learned recently is that 02's loose their ground.this can be checked by clipping a temporary ground from the 02 to frame. not the exhaust. i got this from the bosch rep.
this is very true on 3 wire 02's but 4 wire also. does that me module control the ground side?
well got carried away again, sorry about the long post
good luck rich

__________________
88 300E White, sold 9-04
88 190E Black 2.3 sold
90 300CE White, Grey sold 12-24-04
92 400E White,Grey Sold 5-06
90 300CE 91K Signal Red,Black Sold 2007
1989 300CE Smoke silver,Tan Sold 1-08
1993 300E 3.2 M104 motor
1991 300CE White,Grey
1994 E420 beautiful car,gone to new home
1994 E320 Smoke an Parchment.Beautiful car