You made no mistake however you are in the presence of sheep.
There's nothing wrong with running aftermarket springs and kyb dampers. As a matter of fact, depending on your goals it may be the best solution. Since you're on a drift forum though, I'm going to assume that you will eventually want to drift, in which case a more aggressive, fully adjustable setup like coilovers would be your best bet.
You were asking about replacing your dampers b/c you needed to. If this is true, KYB's are fine. If you're saving up for coilovers, don't spend the extra money on AGX's and just get GR2's - you can get an entire set for under $200. If you daily your car long distances, this is an especially good idea since it will keep your tires in check and give you a smoother, quieter ride.