It's not a good idea to use a battery of less amp-hour capacity than the OEM. The battery capacity is sized to handle whatever array of accessories you have. Though a lower amp hour battery will work, it will be taxing over the life of the battery. Also, if the battery you choose is smaller in size, then you may not be able to tie it down with the battery clamp, which means that the battery will move around in the battery tray, or in worse case turn over since it is not held in place with anything other than gravity.
Interestingly, I continue to find that the Mercedes OEM battery is the same or lower price than aftermarket batteries (such as Sears Die Hards). The battery for my E320 is about $80 at the dealer.............and Die Hards in the size and capacity of the OEM are consistently more expensive! Go figure! Just my two cents.