The memory in SRS stays through battery disconnects.
Yes, you will need a real scanner not a tinker toy. Too bad Snap-On's MB module doesn't do SRS, their BMW and VW/Audi modules do give SRS and other body functions. I imagine Snap-On will do MBs in the future so keep an eye out as even though they are $3-4k new they can be regularly had for 2-500 on ebay.
Probably the cheapest of the available used scanners that would do MB SRS is the AST retriever. This one is currently for sale on ebay:
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=2467030539&category=43989
The other long time professional tool is the Baum CS2000. See this one on ebay:
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=2468514955&category=43989
He is asking too much though. The cs2000 has been replaced by Iscan and they do use the same modules but the new unit has a larger screen and can graph values. The older version cs2000 should be available around 2k I would think.
I sold my AST for a grand about two years ago. I keep my cs2000 untill I decide whether to buy the factory Porsche module or the newlly released Baum Porsche module. So far I have only needed OBDII functions on Porsches newer than my old Porsche factory tool was replaced in 1996. I can do airbags on Porsche currently to only 1996. I am waiting for Porsche's new tool to be released to make the decision. The question is do I spend 10k or so on the real tool and sell my cs2000 for 2k and avoid the 1.5k Baum module. The difference is $6.5k