As for the sump, make it as big as possible. I prefer to have a sump made of acrylic rather than glass. That way, you can use an external pump, which will be more powerful, and will transfer less heat to the water.
If your tank is in the wall, you won't need to do anything to the sides, as when viewed from the front, the sides will be mirrors. You can test this on any tank, just look from the front and you can't see anything through the sides... not from any angle. The only way you'll see through the sides is to view it from one side to the other.
As for the back of the tank, painting is a good option, or you can get a piece of blue or black acrylic and tape or glue it to the back. I have also used pieces of cloth or felt on the back. The benefit of using a cloth might be that you could use a magnet scraper from the back of the tank, if you will have access from the rear. You will definately want to get a good magnet scraper for a tank that is in the wall. I use the Magna-float, and really like them.
Sonny