The only advice I can give you is do NOT fall in to the trap of buying an "unlocked" Sprint iPhone.
Sprint only unlocks their phones for international use. Of course, your phone would be going international. But the problem is that Sprint requires you to be a customer and have a minimum of 90 days on account before they will unlock the phone for international use.
Lots of people are falling into this trap because unethical sellers are advertising Sprint iPhones as "unlocked." There's a reason most of the iPhones on eBay or craigslist are Sprint iPhones.
You might be best served with a Verizon iPhone which I understand come unlocked both for domestic and international use.
DISCLAIMER: I'm a Sprint customer.