I would use live rock rubble, not bioballs if you use anything down there. A lot of people run sumps that are devoid of any sand, rocks, etc. Some use thick sand, it's up to you. I would not use a DSB (Deep Sand Bed) since it's only a 10 gallon sump as a thick DSB will really suck down the amount of water you can place in it.

The only thing to be careful about with an overflow like that is to ensure it stays primed. If it doesn't, you could come home and find all of your sump water on your floor. To be extra careful I would add a Tom's Aquatic aqualifter pump and hook it up to the siphon spout of the overflow. That will automatically ensure it's always kept primed.

The return pump should be fine, if not a little big. I use a mag 5 on my sump (30 gallon sump, 65 gallon tank). But, I like most of my flow from my in tank Koralias not from the sump return. It's kind of a personal preference type of thing.

As to water, place the amount of water you want in your sump first before priming the overflow and turning on your return pump. You want the water level in your sump to be no where near the top. That is incase you loose power. When you loose power, your return pump will stop, but your overflow will run untill the water level in the display is low enough to not push through the overflow. So you will usually get a few gallons that will flow down to your sump during powerloss. Make sure you test this first and mark your level so you always know where it's at.

As to a protien skimmer, I would find a HOB (hang on back) model, not an insump model. Your sump space will be at a premium so you don't want a huge skimmer sitting in your sump.

As to what model, that's personal preference. Are you the type of reefer who will constantly upgrade/change to a new tank? Or will you keep that tank running for a long time? I ask this because if you are going to be constantly upgrading, it's better to buy big the first time. That way, as you upgrade tanks you don't have to also upgrade/replace all of your equipment. I would get a skimmer reccomended around 65 gallon minimum. If your looking for something decent and priced decently, a Coralife 65 super skimmer will work well (just make sure to set the level right or it will overflow on you). The AquaC have some nice skimmers, but they use the rio pump which a lot of reefers hate. Also, Coralvue makes the reef octopus line of skimmers, and they have a 90 gallon rated OCT-BH-100. While it's large, it will serve you through quite a few upgrades.