3Gs to ipod questions

I'm purchasing an iphone 3gs from a friend of mine for 50$. It does NOT have the original sim card.

My mom wants to use it as an ipod touch until their Verizon contract is over at the beginning of next year so they can come to at&t with us. What exactly are the steps I need to take to make it function as an ipod touch? I know the at&t website says you can use any sim to activate it through itunes. Does that mean I can use my sim just so she can use it as an itouch?

Also, when she wants to use it as a phone, will at&t just give her a sim card to put in it?

Thankyou, found too much conflicting info.

 

Awesome, thanks so much!