1) You're not required to have a data plan with T-Mobile if you purchase your phone outright (and prepaid plans buy your phone outright)
2) You'll need a T-Mobile Micro SIM card. You cannot use your AT&T one. They come in kits and are free after rebate last I checked.
3) Yes, you can have your number ported. You may be responsible for a full month of service on AT&T's side and any potential ETF. But you can.
4) As long as you have an active account, you keep your number. Remember, dollars expire after 90 days (1 year if you're GOLD; $100 = GOLD status). As soon as your account goes to 0, you have some number of days (30? 90? 6 months? I don't know, honestly) to reactivate to get your number back
5) You're welcome -- I hope I answered everything for you.