I have seen an adapter, which converts the 38 pin analog diagnostics plug of my '92 W140 into OBD2:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&rd=1&item=7978501068&ssPageName=STRK:MEWA:IT

Does anyone have experience with that adapter?

And can a pc-based OBD2 scan tool like ProScan be recommended for fault codes and analysis?

http://www.myscantool.com

It does seem like a cost-effective solution, but will it work?