Here are the oring counts/sizes if you just want to R&R orings and not the entire fuel line. You don't have to buy them from MB unless you want too.
OM606.962 o-ring size
The lift pump/feed pump is sealed to the IP with a gasket.
To expound, #149 represents injector #6 and the piece you are asking about is the rearmost plug in the series of return lines terminating at #6.