Best place to buy OEM Mercedes parts, and question about taillight assembly repair.
 

My girlfriend backed into something with their 2014 Mercedes GLK350, and one of the taillight assemblies is shattered. She took it to the dealer, they quoted her about $400 for the replacement light assembly and about $300 for labor. I found the part online for about $350, and the video I watched of the replacement seems like its just removing 4 nuts and unplugging the light then putting the nuts back on and plugging them back in. So the $300 in labor is ridiculous. So i have a couple quick questions:

A. Where is the best place (not sketchy) to order OEM Mercedes parts for newer models?

B. Assuming there is some gasket or some type of seal under the taillight assembly to prevent water getting in the car, would I have to replace this, or if it is a gasket, what are the odds it's even damaged? There is no water currently leaking into the car, and its rained twice since she wrecked it.