Hiya,
You've got some balance to work out, even at this basic stage, but it looks as though others have mentioned them.
My issue while playing wasn't game-breaking but did get annoying. While looking at the upgrade tree, when an item was about halfway up the screen or higher, the tooltip would cover it and I was unable to buy it. Might be worth considering having them locate differently depending on where they are in relation to the item they concern.
Good luck with the project!