Mercedes Benz repair: 97 sl320 140Kmiles:?repair manual ?O2 sensor, oxygen sensors, oxygen sensor

Question
I bought a 97 sl320.  The dealership inspection revealed a faulty Oxygen sensor.   The engine light comes on when it is raining.   Local mb capable mechanics: "$500 parts"  "couple of hours in labor".   

Okay, what is the best repair manual to buy?

Which O2 is more likely to fail?  Pre catalyst or aft.

Is this the only forum for "how-to" and "what-not" pertaining to mb repairs.

Thanks for your help.  You seem brave citing 59 MG expertise.

Answer
Best repair manual = MB Tech Manuals from MBUSA cost $19.99 plus S&H
http://www.mercedesshop.com/shopforum/showthread.php?t=142445

Replace all of the Oxygen Sensors at the same time.  

The best Mercedes-Benz DIY forum =  ShopForum = http://www.mercedesshop.com/shopforum/index.php

Shed enough blood on the older MGs and you learn.

Vehicle: 1997 Mercedes Benz SL 320
Part: Oxygen Sensor
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Oxygen Sensor    Year: 1997-
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Oxygen Sensor    Located before catalyst.

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=merc...