There are 2 advantages to a SUMP, 1 you can hide stuff in it so its not in your display, and 2, added water volume for better stability. For #1, size doesn't really matter as long as it can physically contain your gear and will allow for power outages. For #2, bigger is much better. In either case a 20g will be fine for most people with a 75g display, but as above, go with the biggest you can. (make sure you have room to work on it under your tank, trust me on this one
Personally, I don't like the setup you posted for a couple reasons. It has has a "filter media" section. This will require lots of cleaning to keep it from become a nitrate factory, or worse, clogging and flooding your floor. It also will be a poor refugium. I know right now you're looking at a sump, but the typical progression (inside your head) goes something like this.
"Wow, what a great tank I have. Too bad I can see my heaters and powerheads... Maybe I should build a sump to hide that stuff." Then, "Wow, that really cleans up my tank! Hey, what a great article on refugiums providing all the benefits of a sump as well as natural nitrate reduction and a constant supply of live food for my fish and corrals. That sounds great, I should convert my sump into a 'fuge!"
So if you're going with a sump, might as well as design it as a 'fuge. You'll probably want it later. It won't cost any more money than a sump, and the diy
'er will thank you for limiting his projects. BTW
, with a tank or any other straight wall container, you can build one in about an hour, not counting curing time for the silicone.
For your last ?, you can plumb in ball valves and/or check valves to close off the system when you want. Personally I just shut off my return pump and wait for my overflow to stop backfilling.
Hope this helps.