If you can then I would do HHN on a different day instead of doing HHN after a full day at Universal. It's not a deal breaker but it would be better so you guys aren't as tired and can stay late.
You can wait and buy your tickets at Publix, that's what we do. Benefit to buying before you fly down would be to guarantee your tickets just in case that day sells out (unlikely), and sometimes at Publix if you're attending HHN later in the year it can be harder to find the cards in stock as they get a shipment of them and when they run out they run out. Usually its the Fri & Sat tickets that sell out first at Publix. I believe it was last year when we went that we went around middle/late Oct and we checked our local Publix, sold out. Drove up to Orlando and checked two other Publix stores, both sold out. So we ended up just going on our phones and buying tickets through the Universal website.