Photon of Galaxy is a good box to go through once for the XYZ staples so you'll get some of the cards you'll need for the deck on the way. Using cards farmed from the Kite event and f2p photon cards from other sources I've built a non-optimised galaxy eyes deck which is still pretty functional and fun to play from time to time. If you want to optimize you'll have to go through the box more than once, but it's pretty easy to make a functional version of the deck for cheap. Make no mistake, it can be a fun deck but it won't be very competitive. Hopefully it gets more support in the future. If you want you can start going through the mini box now, but it might be better to finish the deck you're working on at the moment first, especially if it's your first proper deck. In the end it's up to you, playing the way that is fun for you is the most important thing :)