I think someone may post before me so sorry if this is a repeat:
But I shall answer them according to what I know.
1) get a hang on back overflow box. Eshopps are the best in my opinion.
2) The intake isn't a problem because the overflow will stop overflowing if the level changes. (bad explination, just don't worry about the intake part)
3) I would just go with the 30g tank. The baffles depend on how you want to lay it out.
4) the main thing is to make sure the pump isn't rated for more than the overflow. Make sure the overflow can handle a little more than what the pump can put out.
5) yes all of that goes in the sump, from what I know, bio balls aren't the best bio filter. I plan to have my sump mainly open and maybe build a refugium into it.
6) A standard sump has a minimum of a skimmer and a return pump. you can add in a mechanical filter sock or a bag of carbon if you need to.
7) the auto level control (auto top off) is good for keeping salinity constant. I would suggest building your own if you have the tools and supplies to do so.
Hope that helps.