Daemonhunters do need a new Codex, but not because it's a bad army-- quite the contrary, really-- it's because the way the Codex was designed was intended for the tail end of 3rd and the start of 4th edition, and the feel of the game has significantly changed since then. With the way armour works right now, Daemonhunters are severely crippled-- they have very little to actually crack heavy armor, and while that makes a degree of sense, it's still a major flaw in the army for the sake of general playability. The Codex is also intended to be used as a suppliment to Codex: Space Marines and Codex: Imperial Guard, and not really as a standalone book, although it's capable of being used as such. GW has gotten away from suppliment books, and has focused on every book being entirely self-contained instead.
Model-wise, the Grey Knights could be plastic if someone were to be clever about how the storm bolter arms were cut. If the storm bolter is attached to the arm in such a way that you can't easily remove it and replace it with a bolter or bolt pistol, for example, then you won't have to worry about people doing what they do with Chaos Daemons and double-dipping the army for different things. Games Workshop's current marketing model is to have kits be relatively interchangable, though, so I'd imagine they'd make it so that you could take individual bitz and make some really cool regular Space Marines using the Grey Knight kit via a kitbashing effort, though.
There -is- a plan to eventually move that range to plastic, but the Inquisition is being redone from the ground up, much like the Dark Eldar-- the concept of the army has to be reexamined to make it work in the current environment, and that takes time.