Check out the special thread for Fluval Spec owners for some ideas.
I would hesitate to use a black background as my Bettas Ted to flare excessively at black/dark background and when I wear black clothing. The background in the photo is a light silver "grass/hay pattern" plactic placemat from IKEA.
50W is fine, I have no problems with Aqueon, you might be able to hide it in the same slot where the pump is. Other 50W that people recommend are Eheim, Hydor.
My Fluval SPEC V has no problems growing low light plants, Mario moss, giant hydrophilia, crypt, pgymy chain sword, wisteria. It is the nitrate that got my wisteria in trouble as I learned from here. In planning your plants, research and make sure that the plants are compatiable in temp and light needs.