The most important dimension is the overall height of the disc stack, within certain tolerances. This dimension helps set up the proper engagement of other parts in the hub. Second you have the number of alternating clutch discs and brake discs. This does not have a bearing so much on the overall stack height as it does on braking surface area (if you had all one-type of disc, you could still produce the appropriately sized stack, but you'd have minimal braking power). The brake converts kinetic energy to heat through friction over this surface area.
With the disc sets you see the thin discs and the thicker discs. The thinner discs produce a higher braking friction area. In theory this should be a better option, but in reality, the thinner discs are more prone to shearing and cracking. I've fixed several sets of the thin discs where at least some of the discs have cracked or sheared. In fact, this appears to have been quite common. The thicker discs, while giving less braking surface area within the stack, are generally more durable.
If you are dealing with any kind of real hills, I suggest the addition of a front brake as well. A drum might be sufficient in that regard, but I prefer a good caliper brake in front. If you're in a flat area and riding casually, you might get by on just the coaster.
I like the New Departure brakes, but my experience is the later Bendix, when adjusted properly, blows them away.