To unlock higher levels of upgrades, keep repairing/creating the category. Eventually, it will level up, and you will either get a stronger version of something or have an unlock (Barrier level 7 for silverback... I can't remember what the sentry was).
Upgrades do save, so you keep your levels... i.e. when you restart Horde mode again, you'll still have your silverback you unlocked last time, etc.
Lastly, if you want to see your stats in Horde to figure out how much money you have left for a level up, go to your main status page (Stats and Awards page) and open up your player stats for Horde. Voila.