Squirtle actually isn't used for attacking. Blastoise maybe but only if you're in a pinch. Keldeo and Black Kyurem EX are the attackers.
As for sets, Keldeo EX is from Boundaries Crossed and Legendary Treasures. It was also a tin Promo. Black Kyurem EX is from Plasma Storm, Blastoise is from Boundaries Crossed and Plasma Blast and is a Secret Rare in Plasma Storm, the best Squirtle is from Boundaries Crossed, Computer Search is from Boundaries Crossed, Dowsing Machine is from Plasma Storm, Superior Energy Retrieval is from Plasma Freeze, Professor's Letter is from X&Y, Ultra Ball is in Dark Explorers, Plasma Blast and Flashfire, Professor Juniper is from Black & White and Plasma Blast, N is in Noble Victories and Dark Explorers, Colress is from Plasma Storm, Shauna is from X&Y, Skyla is from Boundaries Crossed. I would recommend buying Plasma Storm packs since you need so many cards from there, also Boundaries Crossed and Dark Explorers. For the rest I'd say stick to singles.
I'd be happy to answer any questions, TCG or not!