1 I don't think a calculator is allowed other than a CRP whizzwheel
2 Depends on your flying club, ours is free.
3 There are 3 or 4 different sets of papers for each subject, if you fail you will get a different set next time.
4 Dont know about Airquizz, I recomend the PPL Confuser (£20) it has all the current questions & answers though sometimes very slightly different wording, probably 100+ Q & Ans per subject. If you get 95% on those you are more or less guaranteed (sp) a pass.
5 Past papers are not much use, the Confuser is worth every penny.
Good luck
edit:- I aggree with Matelot Who is teaching you?