Will an unlocked 32gb iPhone 5 work with Straight Talk?

Right now I'm currently on AT&T GoPhone $25 Unlimited messaging w/ 250 calling minutes plan. My phone is an AT&T Fusion. Around Christmastime, I'm hoping to upgrade to an iPhone 5, but I do not want to be stuck with a two year contract. So I've done some research and saw that Straight Talk has an Unlimited Text, Calling, and Data plan for only $45 a month, which I can afford. Then I looked at the prices of their iPhone 5's and I about fell over, $700! So I decided I would buy an unlocked 32gb iPhone 5 (I haven't bought it yet, but I've seen some practical prices) on eBay, and then buy the Straight talk sim card to put in it. But as I looked up ways to do this, I saw that some people say you can, some say you can't. So here's my questions:

Are unlocked iPhone 5's from eBay compatible with Straight Talk? If so, do they need to be compatible with T-Mobile, Verizon, At&t, or any other company?

Could I use my old sim card and phone number from my old AT&T phone and put them in my unlocked iPhone 5?

How would I know if the unlocked iPhone was a GSA or CDMA? Should I get a certain one for it to be compatible for Straight Talk? If so, which one?

Would I ever be able to use the same iPhone on another phone company (if I ever decided to switch)?

Anyone who has done this, was it simple or complicated? How's Straight Talk's reception, and data speed on iPhone's? Is Straight Talk a reliable company?

Please be specific as possible, I'm very well confused!

Why is everyone so concerned about a contract? Its not like you are not going to use it for 2 years.

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