Buying a locked iPhone 5

I'm planning on getting an iPhone 5 from the United States ( I live in the uk) so the thing is, I plan on buying an iPhone 5 from apple which would be locked to AT&T and later unlock it.problem is, I'm not sure how I should buy the iPhone, as in what plan, and does it matter what plan I have ( since I want to unlock it anyways?)
so questions: 1.) does it matter how the AT&T plan is set up, if I want to unlock it later?
2.) when I get the locked iPhone in my hands, is it locked in a way that it prevents me from any access ( f.ex.finding out the IMEI number)?
3) when at what point does the at&t plan begin, does it begin as soon as I get the phone?
4.) are there any certified or positive 100% unlocking cervices for iphones?

additional info: I plan on buying the phone when I visit next summer, wondering if there are there any things I should consider before buying the iphone, details I should be careful about?
- any help would be greatly appreciated thanks!

Added (1). My dad lives in the US, so when i go and visit him i'll buy the locked iphone there, and I'll try and unlock it there

How long do you plan on visiting? If you are going to be in the States for less than 6 months, I would definitely not get an iPhone 5 at all! In the US operators like AT&T, Verizon, etc… Make you sign for 2 year contracts. During that time if you want to cancel your contract it will cost upwards of 175 USD.AT&T I believe has to unlock the phone themselves, and I believe you have to have the phone for 6 months. The employees at ATT will be less likely to be lenient with unlocking the phone, seen as you are a first time customer. I would recommend getting an inexpensive Android phone now, activating on TMOBILEs no-contract network for 30 USD a month, and then when you go back home, get your iPhone 5.