Josh, The battery is only two years old. Laura
When not starting in P, did you try in 'N' yet? If handbrake interferes, block the wheels if not on even ground.
Another thing to look out for are fully or partially 'corroded' ground cables (often woven or stranded) from the battery. You usually see a bit of white residue (an old on the road trick was to pour coca cola over the cable. The acid eats away tha corrosion, but it will look quite messy).
Before buying a new starter, check whether the voltage is within prescribed parameters. Frequently a new voltage regulator is the culprit and much cheaper.
The link to the Mercedes factory manual should be in the stickies on top.
Daimler Benz made different spec versions of same model/chassis cars for different parts of the world. So, to get the correct advice on an international forum, location can be important.
Please click 'User CP' and complete your profile with at least a general location.
Search the forum archive with your topic via the 'google special search' box on top, until you get used to the vB