Pretty much any rear disc will be fine so long as it fits. The differences between them are so minor, it's hard to justify spending a lot. So, if you can get a great deal on a used Corima, go for it. Just be sure that the cog threads are still in good shape.
Generally speaking, 3-spoke wheels post better aero numbers, but 5-spoke feel stiffer. If you're small, a 3-spoke should be fine. If you're bigger, you may want a 5-spoke. (That said, supposedly the newer Vision 3-spoke is pretty stiff.)
In terms of options, Mavic, Corima and FFWD are great 5-spoke options. And for more budget friendly options, look at Walker Brothers.
Be careful with generic 5-spokes. I've heard some great stories and some terrible ones.