Personally I'd say if you want to play competitively start by getting 2/3 Keldeo tins and 3 Black Kyurem Boundaries Crossed decks. That would be a good way to get a very rough Blastdeo Deck going on. After that, trade/buy singles to get what you need.
Would you say the Blastdeo deck would be a good tournament deck to try start off with ? I'm just getting back into the game myself and I'm struggling to know where to start. I'm picking up a theme deck or two just for the energys and a few boosters or tins depending on funds.