A note on putting an Optima battery into a Mercedes. If you can dig up an Optima 34R instead of the standard 34, the posts will be in the correct position for the Mercedes cables. The battery comes with a couple of adapters. If you drill out the driver's side battery clamp stud and put a long bolt through the new hole, you can use the type 78 (looks like a 'C') adapter hold the battery in. You will need to add an insulator to the '+' cable as the OEM setup uses a cover on the battery.
Also, after using Mercedes's free jumpstart service (any Mercedes anywhere in the U.S. 1-800-For-Merc) at a park and ride, I installed a Priority Start device. It's an electro-mechanical switch that will cut off the battery if it drains down to 11.5 volts with the engine off. It's saved me several times as the dim dash lights don't exactly shout that you've left your lights on.
[soapbox] However you secure the battery in the tray, make sure it's secure. I've failed several cars during tech. at autocrosses due to the battery being loose. A loose battery is not a good thing in an accident.[/soapbox]