3G network

Hi, i will be traveling to US and getting a pre-paid sim card at the same time.

1. can i use my iphone 4S (from singapore) with T-mobile?

2. thinking of getting the unlimited plan. however, my iphone 4S cannot utilise the 4G network, will i be stuck with 2G or will it automatically join the 3G network?

3. If i get the $60 plan instead, what happens after 2.5G of 4G data? will my iphone 4S be using the 3G network or 2G network?

4. what are the charges if i make a international call back home or a international call is made to me?

5. where can i get the sim card?

Thanks for any advice