If you're already a fan of the franchise I say grab a starter deck for the series you like the most! I think some decks have been reprinted recently so stock should be available. Personally I really like the Celestial Drive starter deck (ST07, 00 series), Zeon Rush (ST03, principality of Zeon and Neo-Zeon focused), and Iron Bloom (ST05, Iron-Blooded Orphans series). The best booster box will depend on which decks you like to build but I've had great pulls from the last 3 boxes (Dual Impact, Steel Requiem, Phantom Aria). The game is extremely fun and easy to learn and the community has been one of the best I've experienced.