1) It depends how you plan to do your flying training: If you propose to do all your flying in the space of 4 weeks or so then it is best to have got your ground exams out of the way
2) There are 2 sets of books to choose from they are the Pratt ones and the Thom ones. Go for whichever set you fine more digestable or take reccomendation from your flying school
3) Its up to you when you get your own headset if you do. I would reccommend you only get one when you think flying is for you as for which make to get its a personal choice but if you can try before you buy I suggest you do.
4) As for ground school training organisations - there arnt really any for PPL just use whatever your flying school has on offer and what suits you best. Some schools do group groundschool others offer one to one. You may even prefer to teach yourself.
Good luck whatever you decide.