I recently replaced the battery in my '94 E320. The old one was the one the factory put in, but, after 5 years, it gave up the ghost, and refused to take a charge. I suspect they only last so long. It was a Varta brand, with MB logos, and a part number of 56219. I replaced it with a slightly larger battery, but also a Varta with MB logos, part number 124 04646. The battery tray in my E320 had two different bolt holes which allowed it to take the two different sized batteries. It seems that it would be possible to put in an even larger battery if you had a mind to.
I also had a look at some non-OEM batteries. Sears makes a DieHard that would fit. It was perhaps 20-25% cheaper than the OEM. Neither size is unusual, so most battery makers would have one that would fit. It is up to you whether keeping your Benz original is worth the extra $$.