Noob regarding SQL 2014 Availability Groups

We've set up a new SQL environment (call it Primary), and want to use Availability Groups/Always On to our DR equivalent at some point.

I haven't set up Availability Groups before. If we have already set up our primary SQL server, can we later add a DR SQL server and enable Always On in primary and DR later? Or does this need to happen earlier on in the process, like at installation? Any other limitations I need to be aware of?

Also, for 2014, Enterprise edition is required, correct?

Thanks,

Scott