The type of plants would depend on your lighting, can you post the details of that?
Some good low light plants are..
Floating - Dwarf water lettuce, duckweed (hard to get rid of so be sure you want it), salvinia natans, hornwort
Attach to driftwood/rocks - Java moss, java fern, bolbitis, anubias
Planted - Anacharis/Elodea (can also float), amazon sword, aponogetons, cryptocorynes, vallisneria, banana plants (I think you just set them on the substrate and they'll root themselves?)
There are more that I'm not thinking of right now.
With those you are fine with gravel although I prefer sand substrate. Fish like corydoras & loaches prefer sand as well if you plan on any of those.
What sort of fish are you interested in? I imagine since you were looking at discus you like color. The rams & apistogrammas already suggested are pretty colorful.
Peacock gudgeons are also colorful..one of my favorites. Small, peaceful, relatively easy to breed.
Perhaps something like marbled hatchetfish for the top, some sort of tetra or rasbora for the middle (I like espei rasboras), some corydoras or khuli loaches on the bottom (if you get sand) or maybe whiptails, and then peacock gudgeons or rams or apistogrammas or honey gouramis as a centerpiece
