new and looking for help

Looking to trade for an iPad2 3g, do I need to look at the SIM card and se if there is a balance with the phone company

or does it not matter since there is no attachment much like you would get with sprint?
and if so do SIM cards cost alot?
also is there any thing special I need to look out for?

kinda need an iPad for work (3g), as the app they want me to use on is available for Apple