How would i start a upgrade unlock system (newbie here)

me and a mate are making a tower defense game and have had an idea to only allow the upgrades in game once they have been purchased. so it adds a bit more depth and a system to make the user wanting to keep playing to unlock all the upgrades.

any suggestions would be useful

Keep a global list of booleans for each upgrade. When you buy an upgrade, make the bool true, when you go to build something, it checks the list to see if the upgrade has been bought