What OS do i need to build to IOS

So I have been working on a project in Windows for a couple years now. I’m finally ready to try and build it to IOS not just Android.

From what I understand I need to buy a mac to do this I’m looking at a refurbished Mac Mini With OS.1.1 El Captain that’s within my price range.

My question is if El Captain doesn’t run any higher than X code 7 is that going to be a problem when trying to build to the newest I phones?

What features am I missing out on but not using X code 8 9 10 and I assume 11 will be out soon?

Any information is very useful to me. Thanks in advance!

you should be able to upgrade it to Sierra no problem. mac OS upgrades are usually free or cheap.
you’ll need maybe a couple of test devices with different aspects. + lightning cable.
Unity and Xcode.

Also sign up for an Apple account, and sign up as an Apple developer $100 per year. Apple Developer Program - Apple Developer
That allows you to use iTunesConnect and release titles to app store.

Maybe something like a HockeyApp.com account to distribute builds (you might already use this for droid)… or use Testflight.