AEther Revolt has Walking Ballista ($5.97), and Skyship Plunderer ($.98), and perhaps you should look at Tezzeret the Schemer's -2 ability that changes Walking Ballista's base power and toughness to 5/5 instead of 0/0.
(AEther Revolt update)
Walking Ballista
Skyship Plunderer
Tezzeret the Schemer
If you revisit the idea, AEther Revolt should make this MUCH easier, er efficient.