You can easily make a plastic tank that will withstand pressure, the trick is trapping the air cushion to absorb expansion. The baffles are there to create an air pocket, period. If they were there for structural reasons, the holes could be much larger to facilitate filling. Because the holes are so small, it's important to slowly add coolant until it comes up to the filler neck and stays there, otherwise you'll end up low when the coolant finally settles out. You hear all the time about people "burping" their system after refilling. Who would have guessed that the burp lived in the header tank?

As it happens, I'm currently running G05. And yes, if there's actually an SCA packet in there, then the coolant chemistry has to match.

The new tank came from a 500E, which happens to have a tank that's dimensionally identical to a 190D turbo (but not to an NA 190D). I don't know what model you drive, but I believe all 190's have level float switches, which operate a dash light.