Barbarian is similar to Hoard. Of course Hoard has the advantage that it is a a Treasure and that you can regularly play it (i.e when you buy it and play it the first time even without getting a Victory card and a Gold this is not a big issue) whereas Barbarian could sit on the Tavern mat for too long. You could also miss the "tipping point" in a game without extra buys where you did not go for Barbarian early on, generate in all turns more than 3 and wanna buy something better.
So 3 seems like an OK price. I'd also test it at 4 though.
Exchange is good and IMO the best implementation of your Ruins junking idea that has been present in several of your cards.
About
Progress, the other player always have an incentive to flash, even in a 3 or 4 player game.
So most often this is worse than a Lab. You only get a Double Lab if, in a 3P game, both other players and you flashed ... but this is just Governor with the draw option and your Double Lab is mitigated by the fact that the other two guys could drew a card.
Of course Lab is so good that a card which is virtually always worse than Lab is OK for 5. But I cannot imagine anyone going for Progress with Lab in the Kingdom.
About
Philosophers, it is the first version of Steward but I think that it is borderline viable due to the overpay. Probably still a fairly bland card but that's not the end of the world. With the overpay you can easily get 2 for 4 and this might create enough flexibility in Kingdoms without drawing or extra buys or even without a village.
Steward: The first version of this gave you a choice between +2 Actions/Buys/Cards/coins. It wasn't interesting enough, so I traded in the Actions and Buys for trashing two cards.