I'm saying the base HF unit is a piece of junk and in the long run not a good investment at all... as in a waste of money... that makes it expensive at any price...
Impeller and motor failures are a common occurrence w/ them... so do you replace or repair..
Hmmm, all I've ever read is that it' a great value. I've never read anything about failed motors and impellers.
I've had mine for about 5-7 years, and it's been great. Best $135 I've spent.
It's more than adequate right out of the box, with no upgrades at all.
People will tell you that you must get a canister filter. My experience is that the bag doesn't allow any dust through it.
To your original questions.
You want to run 4" all the way to the machines. If possible, replace 2-1/2" ports with 4" ports, and collection will greatly improve. 2-1/2" hose will not work well with a dust collector.
I'd actually run a 5" main line, as that's the size of the intake port on the HF.