Depp would be perfect IMO, the classic Strange I remember had those trademark gray sideburns, so the age fits. Plus, ya gotta think after all those years of medical school, residency, and establishing a practice as a successful surgeon, followed by the car accident, recovery, and subsequent wandering the world and studying the mystic arts for years, you'd have to think the guy would have a few years under his belt by then.
They did name drop him on SHIELD, or maybe it was Winter Soldier, so the seed has been planted. I agree this could be one of the coolest movies ever, if done right. (IE: stick to it being actual magic, and not some interdimensional mumbo jumbo) Dormammu would be a terrific villain too, not to mention Mordo.