The best alternative to new battery for the R320 ..at B.J's.. Exide batteries for just a shade over $ 100.00.
For the replacement of the auxillary battery.. Amazon for $14.99 Part number original 4039 .Battery by Mighty Maxx.
The most common reason for the battery light to come on would be a failure in the charging system. It could be the alternator, but with the cost involved in replacing the alternator, I would recommend having the problem diagnosed before simply replacing it.
Because of the age of your car, you may need to replace both the auxiliary battery and the main battery. Just replacing the main battery does not necessarily solve the problem. Just this year April 2012, I first replace the main battery and still got the battery light and then had to replace the auxiliary battery and the problem went away. The mechanics checked the alternator in the alternate was fine but there is a limited life to both batteries. The cheapest battery that I could find that was a exact replacement for the Mercedes battery was obtained at batteries plus.