I like the concept, though it seemed like every time I saved up to buy a helicopter the enemy bought one too. Whenever you buy a unit there seems to be a percentage chance that the enemy will receive an identical one of what you bought. I think what I would have preferred is for the enemy to have an income, like you do, and perhaps it buys upgrades randomly when it can. Or maybe picks an upgrade randomly and then waits until it can afford it. Or something like that.