Fishroes --
In answer to your questions, the Flex 3401VRG for sure will be able to use the 5" backing plate and a couple of smaller ones..
When I bought mine way, way, back when they first came out, it came with a 6" backing plate, and a really nice bag, and me a a bunch of other people immediately cut the backing plate down to around 5" so we could use the 5.5" pads we all found to be the best for our needs.. Flex eventually made the 5" backing plate available, and later still, came out with couple of smaller backing plates for it..
The counter-clockwise rotation took a minute to get used to, and vibration wise, I don`t know if it is better or worse than other machines because I don`t have them to compare it to..
This model 3401VRG Flex does have more forced rotation than the random orbitals of the day, and I don`t recall it ever stalling out on curves, etc., as I hear machines do today..
The thing to remember about all these machines that are NOT direct drive like my Makita is that they were developed for those that did not want to or were not inclined to want to learn how to handle a big 10+Lb Rotary and the learning process that goes with it..
Reading your post, I think you might be more inclined to want a Random Orbital machine vs a direct drive Rotary, and that can also be a great choice, as many of the top guys on this Forum alone produce exceptional work with those machines..
No matter what you pick for your machine, it will always be about pad rotation to get correction and finishing and in a timely manner..
I also have had a probably 20-year-old Porter Cable that I eventually gave to a son-in-law to learn how to clean up his cars, and he has gotten very good at it..
Eventually I purchased another Porter Cable that I mainly use with a smaller 3" pad for polishing all the glass and sunroofs of vehicles now..
Of course, if the glass is extremely etched, etc., I put a small pad and extension on my Makita and clean up the glass really fast...
Good luck with your research !
Dan F