While you're probably ok getting them before certification's done, there's no guarantee that your pre-cert AC devices would work after the standard's finished. And from what I remember from when N was first coming out, first generations were pretty poor in the ways of performance.
N's capable of 130Mbps (16.25MB/s). What kind of application are you looking at that you'll need something higher?
Edit: If you get devices of the same brand then you should be fine. Should have mentioned that before.
Should be able to pick up a decent n for under 50 bucks nowadays. Not a huge investment in the interim.