IMO a painted black background looks better than anything else, BUT with the tank already setup and against the wall you may have to settle for a solid black plastic background from the LFS.
For filtration in fw I think canister are the best way to go. Their mechanical capacity is beyond any open filter since it is a closed, pressurized system. However, since you already have an FX5 (which I have seen run heavily stocked 210s as the only filtration), I think a sump is a good idea. I wouldn't drill it, that would require a lot of work and risk. I would just add a HOB overflow box. Use the largest tank you can fit under there as a sump. One of the biggest benefits of a sump on a tank like that is, depending on how it is setup, you can use it as a place to put fish that are too aggressive for the main tank or are getting picked on.
