OK guys, challenging question here... INTL usage

Hi guys,

I'm based in Europe and looking to buy two iPhones when going to the U.S., which will be soon. The phones will only be used in Europe. One will be an old 4G for my brother, I'm looking for a 4S or 5.

But, all possible options get extremely confusing, so I thought I'd ask you more knowledgeable folks for some well needed help.

So here I go:

1) What options do I have for buying an iPhone 4?
We basically use pre-paid SIM phones (GSM network) in our country. Which 4G phones/editions would work?

a) CDMA (Verizon/Sprint) is out of the question, even if they are unlocked/flashed/..., right? Is factory unlocked the only possible option? What about AT&T?

2) Same question, but for iPhone 4S and iPhone 5
I read somewhere, but the source was dubious, that it will be IMPOSSIBLE to use a (/any) US iPhone on a EU sim-card GSM. Can you confirm this?
Please note that I don't care if it's jailbroken/unlocked/factory unlocked/unusable in the U.S./....
I don't care for any of that, as long as I can use it in Europe with pre-paid SIM cards (excluding T-mobile and O2).

Can't be hard, can it?

Thanks a million guys.