Having the right trainers is important. Key trainers to look out for are Bebe's Search, Roseanne's Research, and Team Galactic Wager. The rest of the supporters are pretty much just draw. There are some interesting new Supporters coming out in a few weeks in the new set Legends Awakened too. To get some good trainers, I would recommend buying singles through ebay or google shopping. If Roseanne's are too expensive, you can buy the Electivire theme deck from Secret Wonders and it will come with 2 of them. I think Bebe's Search might come in some Mysterious Treasures theme decks, but I don't know if it does or not. Rare Candy is another important trainer that all players need to have some of. It doesn't come in a theme deck, but it's been reprinted a few times, so it shouldn't be ultra hard to get some. Night Maintenance is another important card. Might come in a theme deck.
Most of the good special energy is being rotated, so don't worry about them. It might be a good idea to get a good amount of Basic Dark and Metal energy. Diamond/Pearl theme decks (first DP set) seem to be the best places to get them, and I think one of the Great Encounters theme decks come with basic dark (and/or metal) energy. Call Energy from Majestic Dawn is a special energy I would recommend getting. I feel it will be very useful next season.
There are some Pokemon that could be useful in every deck, like Pachirisu (GE) and Phione (MD), but there are trainers and energies that can do the same thing. Generally though, there aren't a lot of "staples" pokemon-wise. Maybe look out for the common Chatot (MD).
LA (Legends Awakened) looks to be a decent set, and there should be more Lv.Xs in a box since the set has 7 of them in it. I would recommend buying some of LA. Tins are always good deals. There are new Tins coming out in September that will have Mewtwo lv.X and Rhyperior Lv.X in them.
I am sure I forgot to mention some cards, but I am sure others will mention them.