Hi Mark, welcome to the forum and it's good to hear that you are considering one of the best 'sleeper' that is rarely remembered.
I get 40+ mpg on a steady run and have averaged 35mpg over 2 years.
Mine is 58 plate MK3 and the boot isn't any bigger than a slightly earlier pre-face lift. As already mentioned the boot is small which pretty much works for me and the mrs, I have roof bars and a rear box if we're travelling more than 2 up, although I've not needed to use both at once. If boot size is important then I believe the MK4's get a bigger boot but I haven't seen one in person to compare, the later cars also much more expensive.
I have an AUX-in phone jack in my center arm rest but I rarely use it. I don't have any cassette tapes, so I've never used that. The ML speakers/etc great tho
As you've already found, the Hybrid batteries can be repaired cost effectively, there's also been a few reports of a pump failure that can be fixed cheaply rather than buying a new replacement.
I've grown to really enjoy the CVT gearbox and the drive train gives one mighty surge when needed, I've never felt it's been short on power!
Have you driven one yet?
Good luck in your hunt and do keep us updated on your progress.