I'd get as good a dado set as you can afford. A 6" is all you need. As Hilton said, the manufacturer uses the optimum number of teeth for their specific sets.
Making box joints with a dado set can be problematic. I know of no manufacturer that produces a set that yields truly flat bottomed dadoes. There might be one, I just don't know of one. Mine is out to a sharpening shop, where the outside cutters will be ground to match the chippers.
The Freud set that Hilton referenced will give you flat bottoms. It is OK if all you will ever need is 1/4" and 3/8" dadoes.
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