As you level and items become more powerful, the cost should increase greatly at shops and the chance of finding them as random loot should drop so you have to purchase. It would also be nice if you could go to a Jarl and donate money for repairs around towns, Winterhold for example or even purchase a business or Inn, not just invest, which has an associated cost for running it.
Personally I would hate to see loot % decrease. For me finding cool stuff is one of the delights of exploration. None of my characters ever buy gear. They use only what they loot. I'd be against any change that would force my characters to buy any of their gear.
I solve this problem with the http://skyrim.nexusmods.com/downloads/file.php?id=2762 mod. With this mod my character couldn't afford Breezewood until level 19. She was finally able to completely furnish the house at level 28. And then she spent the next few days travelling by foot because she couldn't afford a carriage!
I like your idea of donating money for repairs though. I think it has the potential to be a great mod. For instance, Whiterun could revert back to its pre-war state after the player has donated enough money to repair the city.