Right now you can sell stuff to any NPC, so if you're having inventory problems and want to clean out some junk, consider that. You don't really need access to a retainer until you're creating and selling finished goods as a high level crafter. Don't sell your older gear that you've worn to NPCs, though. Hold on to it so you can turn it into materia later.
Retainers are unlocked as part of the main quest scenario. I'm not 100% sure what level, but someone else probably knows.
As for the storage chest, I don't know either.
This and the retainer opens after you finish the 3rd dungeon, Copperbell Mines.