Really great job!
One wrinkle that I think could add a lot of strategy and complexity to the game is limit the Upgrades so any given player cannot have upgrades that total more than their total tech level. As I read it currently, the total tech level only limits when they can buy the upgrade. The way (as far as I understand) it works now, the players will tend to homogenize as they trade upgrades but don't lose them.
Although that removes the incentive to trade a bit, as you can no longer trade a player for an upgrade you can't develop yourself. hmm....