The main battery is an H8. It is a standard size, very common around the world. You can scare one up at any auto parts store. As SRJ says above, some are better than others. Certainly no need to buy at the MB dealer.

An H8 is an AGM battery - it's a different technology than conventional lead-acid batteries, but serves the same function.

Just swap it out like any other conventional battery. You will likely need to reset the auto-up windows and sunroof, but that's all that can happen.