My 08 R350 makes a load rattle only when I start the car for the first time, subsequent starts the same day it does not rattle. The sound last for just a couple seconds. Thoughts?

The first thing that will come to everyone's mind will be a stretched timing chain. Rattle on first start is the classic symptom. Assume you car is gas and not diesel?

Search the forums including W164 and X164 and even W211. All used the same engines. The M272 is well known for this issue prior to mid 2008. See the attached bulletin from MB that specifies the engine number ranges of engines that are impacted by the balance shaft issue.

Is this something that be replaced at my home with no lift?

Why would the pickup tube seal ring cause a rattle at startup? Timing chain and tensioner I understand, or something in the oil filter causing it to leak down overnight. Or do you mean the seal inside the pump?

Sump off either way I guess. But why that part and not the chain?

The seals fail and cause a lack of oil pressure on start up.