I want to buy an unlocked iphone 5. What do I do?

I want to buy an unlocked iphone 5 for my boyfriend for his birthday. I saw on on craiglist for 400.It is an unlocked iphone 5 from verizon. My boyfriend has simple mobile. He has already cut his sim card for the 4. Should i get the phone since its unlocked or do i have to get a tmobile or AT&T phone? And do I need to buy a new sims card for him? If i do how do i go about that?

The Verizon one will work.