1. I think it is better to integrate the Groundschool with the Flying Training. I think it makes more sense when you can apply the groundschool straight away.
2. Best books are a matter of opinion. To my mind Treveor Thom's cover more ground but in a slightly less clear manner than Jeremy Pratt's.
3. It is worth buying your own, good quality headset, but only once you are sure you want to learn to fly. Best to have had at least four lessons before splashing out on a headet. It's not necessary to have the very best Dave Clarks or Bose - buy on performance, not on name.
4. Dunno.