Be a bit hesitant on buying into the decks using Dragons of Tarkir/Magic Origins power-mythics, the expensive ones (except Jace, Vryn's Prodigy, Collected Company, and Kolaghan's Command) are pretty much useless in Modern, so they're going to crater a bit post-rotation (Which is in ~4.5months). Every once in a while, a Japanese player tries to make Dragonlord Ojutai "a thing" in Modern, but it hasn't really seen any kind of widespread adoption or success. I'd also be hesitant on buying into blue in general, because a ton of its equity is wrapped up in Jace, Vryn's Prodigy; Kaladesh is going to need some pretty damn sweet blue cards to hold up the color when JVP rotates.
If you're interested in the B/G Seasons Past deck, look into the B/G "Aurora" Deck Sam Black played at the last major standard event, it's an evolution of the deck that has better pressure for opposing planeswalkers (as planeswalker-heavy decks are doing very well right now, between G/W tokens and B/W or B/R/W Control), and a hilarious end-game of tutoring for and casting its singleton copy of The Great Aurora. The Aurora/Seasons Past build will also have you picking up some of the better staple creatures of the format that will be legal for over a year moving forward. Kalitas and Tireless Tracker are value-grinding cards, and Sylvan Advocate is going to be a competitive staple for his entire standard run, unless Kaladesh randomly turns it into a 4-turn format.
