Here in the UK, I've listened to too much Andrew Castle and Mark Petchey to be able to imagine what a great commentator would be like. Even if an ex-tennis player was able to articulate clearly the tactics each player was adopting and comment interestingly on the narrative flow of a match the TV channels here would just say "The audience haven't heard of this guy. What is Boris Becker's phone number?"
From what I've heard of the guys near the top of the game Tipsarevic seems one of the most thoughtful and I'd probably plump for him as commentator.
As for a coach I guess you are looking for someone who was able to adapt his game and take advantage of new strategies/techniques rather than someone with a more one-dimensional game who just worked really really hard. I think Nole would be an interesting shout as I don't think Fed would ever consider it.
wait...that was a joke right?........... wasn't it?
As I stated earlier, I'd like to see Roddick as a commentator. He is witty, and has knowledge of the game. What I'd really like to see though, is either Safin, Gulbis, or Djokovic take over from Justin Gimmelstob.
As a coach...hard to tell. Any of the succesful guys now might make good coaches after a decade or so. If they'd do it right away, it won't work. Quite hard to coach someone who you can still beat easily. But if I had to choose, I'd say Ferrer.