Are you approaching this as strictly an investment, a game for mostly enjoyment, or somewhere between? I'm somewhere in between, but primarily I play for the competitive enjoyment, and secondly for capital gains. I personally would use the bid feature on peakmonsters.com to get 5 BCX of a cheaper Chaos Legion summoner to get a level 2. Then, I would put in bids with the remaining DEC for a good leveled up common or rare in that splinter. Good luck in whatever you do!
RE: What to buy?