If I buy a Sprint iPhone 5C and have it unlocked, will 3G work in Europe? Or only Edge? I realise LTE / 4G may not work.
Careful here.
I strongly advise you to consider another iPhone from a different carrier, no matter how cheap the Sprint iPhone may be.
Do not make the assumption that you can purchase this phone and then just call Sprint up to have it unlocked, or to have a third party unlock it. Sprint will not unlock it for you no matter how much you beg or plead and there is no third party unlock for Sprint iPhones.
To get an INTERNATIONAL unlock only you would need to do the following.
1. Establish service with Sprint (no ASL account).
2. Keep service and pay your bill on time for 90 days.
3. Call Sprint for your international unlock.
Sprint will NOT unlock the phone for domestic US carrier use - ever. And they will not unlock the phone for anyone who has not established service with them and paid their bill on time for 90 days.
So, if you buy this phone knowing this you are essentially buying yourself an expensive iPod Touch. The only other way around this is a Gevey, R-SIM, or other type of SIM Interposer. That, of course will cost you more money and is not a true unlock.
But assuming you do get it international unlocked, check the bands/frequencies of the carrier you intend to take it to. That should tell you what you can expect.