Most of the stuff here sounds nice, but I don't know about the Steel and Duralium stuff.
Now, as
@Lecic said, the Steel stuff would be nice in MP so you don't have to grind as much to get the equipment for everyone, but to me, it just seems like giving the Merchant a whole new set of equipment is just... too much. Besides, it'll fill up eight slots in the merch's inventory (unless I overlooked something), and something that's more powerful than Platinum costing less than silver also seems a bit off-balance. Now, of course, if it's so expensive that when farming bosses for the money to buy it, you actually get enough resources to craft Shadow/Crimson armor, it wouldn't really work either, so I'm a bit out of ideas there. Also, Steel is actually just modified Iron, so it wouldn't be logical for it to be that much better, but I know, Terraria logic.
Still, to get back to the issue at hand: I don't think the Steel equipment really fits Terraria. I don't have a logical or rational reason for it, it's just a feeling. Similarly with the Duralium equipement, another whole equipment set just doesn't seem right. Also, there's no aluminium in the whole world of Terraria, but after murdering the Keeper of the Underworld and releasing ancient spirits, the Merchant suddenly has access to Duralium equipment.
Next up, this would pose a sort of balance issue IMO: Up until now, use you activate Hardmode, you quickly go to the Corruption/Crimson to smash some altars ang get mining so that you can be prepared before the biomes spread too much. This way, you could pile some money beforehand, and once you enable hardmode, you immediately get a shiny new set of armor that may or may not be better in different aspects, but gives you a 10% movement speed bonus, which will probably make it somewhat easier to survive in early HM, as you'll be able to smash a bunch of altars faster than the Wraiths can come, all in all reducing the challenge of the first few minutes of Hardmode.
I might have been exaggerating a bit here, but to me, the Steel and Duralium don't quite fit, don't quite make sense and aren't quite balanced. Don't ask me how to balance them though, as I can't really work on something that doesn't seem to have a place where it is.
Also, sorry for the Wall of Text. I tend to get carried away when criticising something.