If you get a waterstone a flattening plate is HIGHLY recommended if not required. A waterstone (regardless of the brand) will wear down and the middle will dish out. That means the middle will be lower than the sides and it can greatly affect the outcome of your sharpening.
The slurry stones from DMT are very small stones used to raise a slurry quickly like a nagura stone. They are not really large enough to effectively flatten a large stone with. You want a larger plate that has about the same surface area of the stone so it will be able to wear down the surface of the stone more evenly and will result in a flat stone when you are finished. The full sized DMT or Atoma plates would work just fine for this and can also act as slurry stones just by giving the stone's top a few scrubs with them.
In order from cheapest to most expensive, here are some popular flattening plates:
140 grit (Might not be completely flat) : http://www.chefknivestogo.com/140grdistflp.html
DMT XXC (Better) : http://www.chefknivestogo.com/dmtextracoarse.html
Atoma 140 (Best of these three) : http://www.chefknivestogo.com/at14dipl.html
Functionally they should all work pretty much the same, you are just getting a better quality plate with each price jump. If you get the cheap 140 grit stone you might see if Mark would be willing to take a quick look at it and see if it's flat before shipping it out. He's a good guy and probably will if you ask nicely.