It's usually a good idea to go back and replay levels on advanced or extreme after you've progressed a bit further into the game, as that gets you free weapons and armor while they're still useful. It also can save on scrap which helps as well. Unless you're doing a LOT of grinding, you generally won't have enough scrap to buy all the best items for everyone. Some of the items have a tendency to last for quite a long time, as well. My general rule is to go back and replay a stack of previous levels on the next difficulty every time I progress to a new chapter. That way most of the items end up being useful until near the end of the game. The items don't have any extra bonuses beyond what's listed in the normal game.

Once you get into NG+, though, uniques become EXTREMELY useful as they can be upgraded into items that are not only more powerful than purchased items, but also have extra effects (like increasing bleed percentage, adding chill, dispelling darkness, adding ranged evasion, etc). Each item gets a different bonus, and they work best if you give them to a character that's appropriately specialized. There aren't any "hidden" bonuses, though, everything is always displayed in the description.