In designing the sump, a few factors come into play. First, how much room do you have available? Most sumps are installed beneather the main tank, but some put their sumps behind or beside the main. Leaving a bit of room around the sump is a good idea, so you can expand with possible reactors and dosing pumps. For now, they're not needed, but you should leave a bit of room on one or 2 sides, about 4".
Heat! If you live in an area that gets real warm, and heat may be an issue, you might want to conside an external return pump for the sump. Than removes a large amount of heat transfer from the pump to the water, but it adds a bit of a plumbing issue. Not much, but some.
Most people run their sumps with 3 sections. Personally, I only have 2 sections in mine and I have combined the skimmer section with the refugium section. Not really an issue I think and the only way I would do it again.
For a sump skimmer, or an internal skimmer, Reef Octopus are popular. I have one, but I will admit I find it a bit noisy. Perhaps others will chime in on what they use in a sump and if there are any issues.