1. if you're talking a phone from a carrier it will come with the SIM card, you can always buy an unlocked phone off Ebay that will work on either AT&T or Tmobile. AT&T gives out SIM cards for free, I think Tmobile charges like $5-10 bucks.
2. the sim card might still work, probably a 2G one and most new phones are 3G, but some still aren't, the number is gone you won't get it back.
3. yes, AT&T will let you use any phone of theirs, or any unlocked phone you have on their Pay-As-You Go service. T-Mobile is a better bet if they have good coverage in your area. They're pre-paid is cheaper, with the exception of no text message add-on (which AT&T offers) that is. Somebody correct me if I'm wrong but T-Mobiles site doesn't say anything about pre-paid text bundles in their pre-paid. AT&T is expensive, I have it and am about to go over to T-Mobile, for the same price I'll get Nights & Weekends added on, AT&T wants $20 extra a month to add 3000 N&W mins. T-Mobile includes them in the buck a day service (plus .10 a minute for day time calls)
Quite a few phones can be unlocked if you know what you're doing. AT&T's offical selection for their Pay As You Go phones isn't the greatest, but you can use a Blackberry, iPhone or whatever phone you wish.