Installing specific iOS version on iPhone 4S

I've been asked to purchase an iPhone 4S for a team of developers for testing purposes. Two requirements that have been asked of the iPhone are:

• it should run iOS 7.0 instead of the most recent 7.1 (don't ask me why, I'm oblivious to the reasons).
• jailbreaking is unacceptable

My concern is: if I get an iPhone 4S with an older version of iOS will I be able to upgrade to version 7.0 specifically or will Apple force me to install the most recent 7.1?

Yes, I've seen some similar questions here, on Ask Different, but they were concerned rather with downgrading to a lower version of iOS (e.g. from 7 to 6, etc.) and thus do not seem to apply in my case.

Solution 1:

This answer can be pretty short and easy:

I don't see why it should be 7.0 but you can only upgrade to the newest version around (this is iOS 7.1.1 as we speak). This is because Apple blocks signing the devices with older firmware files.