I'd Skip the PW. Too many issues with a third party unit. ... Some have been waiting 7 years for a new 100-400L. However, it currently had no real competition, its the best of its kind.
Agree with the 3rd party flash. I waited a few years hoping for the RT's and they did deliver, so I plunked down for three units plus the ST.
I'm one of those still waiting for the 100-400 though It's OK, no rush.