Problem with the w210 chassis is the battery is under the rear seat cushion, not under the hood in the previously typical MB location. The Optima is a group 34, while the standard battery in the 210 is a group 49. Generally the group 34 is smaller than the group 49 - so far, so good. The potential problem arises in that the group 34 is taller - 7.875" vs. 7.1875". Unlike the earlier cars, which have several inches of clearance above the battery to the hood, things are rather tight in the 210. I don't know that an Optima won't fit, but would check things out carefully before going that route.
One other note - there are only two batteries I would ever consider for the 210 - either the Optima or the stock MB battery. Because the battery is in the passenger compartment, it is vitally important that hydrogen emissions be controlled. The MB battery has provisions to vent the hydrogen outside the vehicle - it is specially designed for this application. The Optima is completely sealed and almost never releases hydrogen. I would never use a standard battery which just vents hydrogen to the atmosphere - that's looking for an explosion in the passenger compartment.