Good that you are doing research man! Since you have time and obvious interest, make it an awesome sump! One piece of advice is to build one that can hold a lot of water; in essence, build a big one. You'll keep adding to it over time and the bigger it is, the easier to add equipment, maintain the equipment and most important, helps to create a larger water volume for greater overall system stability = health.
Depends on how you set it up. Short answer is yes and yes. The sock should be underwater, but again, it depends on how you design it. I would absolutely make sure that the overflow pipe terminates under the water level so that it won't be loud, splashy, etc. The sumps that I like personally don't have the overflow pipe directly plumbed into the sock compartment, but rather outside it in its own area, then water flows up and over the sock compartment and back down through the filter socks. Images below will help make sense of that...

How are you designing your sump? Is it going to be a traditional box style sump with dividers, etc? Or the Rubbermaid tub style? Or something unique and different? I'd like to see a thread on your tank setup with sump once you have it planned out.
For the traditional sump style, here are a couple of pictures of a sump where the overflow pipes are plumbed so they are underwater , then water rises up and over into the filter sock section, then from there do what you want with the sump, but as far as this thread is concerned, the images should answer your question about overflow pipes, where the socks are, underwater, etc. As far as overflow design is concerned, if you aren't too far into the tank design, take a look at the
beananimal design. If you have not seen a beananimal design, click on the link, you will definitely want to incorporate the design if you have the ability to drill your tank to allow for it and/or aren't too far along. Also take a look at the S
ynergy Reef overflow or Reef Savvy Ghost overflow. Different companies that collaborated on an overflow but sell independently of each other.
This image is from
Oceans by Design, and since it isn't mine and I don't have any idea who they are (but their sump design is pretty. LOL), I figured might as well give them credit.