Hi and welcome to the forum! There are definitely some folks out there who have converted reef systems to discus, and I think you can even find some old threads that document some of their experiences in the Tank Journal section, IIRC.
If you're new to discus, this video from the forum's owner and administrator is a great place to start: http://forum.simplydiscus.com/showth...TCH-THIS-VIDEO!
Most of us coming with experience in our parts of the hobby (myself included) have found that simple setups are great (often preferable to more complex ones), and that most of the technical ingenuity (if there is any) goes into planning water storage and water changing systems, which ideally are made easy for regular (often daily!) use and are sometimes automated.
Sumps are widely used and are great for discus, and most will say that any biological media of choice is fine to go with. I know some here use Poret foam in staged sheets. But really, anything that allows you to prefilter and easily clean regularly at the mechanical stage is good. You may want to think about a good heater or two and perhaps a temperature controller to manage them as they'll be running a lot. (Some also insulate their tanks to some degree if it's not acrylic.)
The UV is fine especially for water clarity and there's no harm in using it, though most will say there's no need to add one. Since you have it, I would use it.
Maybe others who have dark backgrounds will share their thoughts. It's true that discus will change color in relationship to their surroundings, and will sometimes darken to camouflage themselves with a black background. That can be less than ideal in some instances, partly as they also do this when ill (which could cause you to miss something) and there are some strains ("pigeon-based" that may show undesirable "peppering") against black.
Most folks seem to go with a light blue, white, or off-white, but there may be some who have had good success with black who can speak to this.
General rule of thumb is one adult discus for about 10 gallons... for a big tank like this you can certainly do a large group, though sometimes the budget can be an issue... Discus do well in groups (usually at least 5-6 adults as a minimum) or for growing out smaller ones, 10-12 would be a good number. For larger tanks sometimes it is suggested to start smaller fish in smaller spaces (a grow out tank where they can stick together, feel safe, and easily find food) and move them later. If you can afford to add a large number (10+) of 4-5" adults to your tank that could be a great way to go. It's much easier to get all your fish at once from one source, by the way. If you decide to add to your group later you'll want to carefully quarantine any newcomers to avoid potential problems.
Anyway, hope this gets you started and there are lots of folks who can answer questions that come up as you make plans!