If you're in a powerful end-game guild that farms MC, no, you don't need to craft any Dark Iron gear - you'll get better from MC and BWL drops. However, they got to be powerful endgame guilds by crafting dark iron gear and using it while they worked their way through MC, and there are still guilds working their way up that would gladly buy Dark Iron Helms. I've crafted six of them myself for guildies - I do the crafting for free, but the person that wants the helm provides the mats and comes with me to the black anvil in BRD to forge it. I've crafted the boots for a non-guildie, he was willing to pay 25G just for the crafting, as well as providing the mats and forming part of the BRD party.
The mats are expensive because they drop from random mobs in MC, and most of the guilds farming MC are using the mats to craft gear for themselves. The only ones available are either from guilds who really don't need the gear anymore and are getting as much gold as they can for the mats, or from PuGs, which on my server at least are getting more common. I would expect the price of the mats to drop once it becomes possible to farm MC in smaller groups.
For Aarros - the trainer for smelting Dark Iron ore is Gloom'rel, one of the ghosts in the seven ghost room towards the back of Blackrock depths. You need to take 20 gold bars, 10 truesilver bars, and 4 star rubies down with you and talk to him before anyone starts the script to kill the ghosts. Once you've talked to him, a ghostly chalice will appear next to him - click on it, accept the quest to give up all your nice mats, and talk to him again (that part is important!) - he will teach you to smelt Dark Iron. Since you're only level 52, you probably haven't made it into BRD yet, but you have to go to the black forge to smelt the dark iron, and to the black anvil to forge anything from it.