No idea how to build a sump

I currently have a 180 gallon freshwater tank, formerly it was a saltwater tank, drilled on the bottom with intank corner overflows on both back corners.

Question: Right now I have the holes plugged in the overflow boxes since I'm not using a sump on it...
1. Is the water pressure in the overflow different from the rest of the tank? I just want to be sure, don't want to spring a leak out of the bottom, so far so good. - The overflows are empty, it's sealed around the boxes with no leaks so far into the area with the holes. (What's the best way to seal these anyways?)
2. I have some piping, not all of what I need I'm sure, and was considering setting up the aquarium for a sump...I have no idea how to build one. I have a 20/g and two 10/g I'm not using and was wondering if I could use these for the sump. Also, anyone have any instructions on how to attach it to the drilled holes without it leaking around the pipes, let alone how to set up this type of filtration? I've seen a picture where one pipe emptied into the bottom of the tank under the following: carbon on bottom, ceramic beads on top of that, then plant substrate with live plants above that. The other side of the tank had a pipe feeding into the tank with the water above all the substrates/plants.
3. Also, I've read a couple of places about a shut off so when the power goes out I don't have a flood, can someone elaborate on this?
4. What's better: pumps or powerheads?

I'm sorry if a post like this has been posted, but I've looked online and cannot seem to find one for a freshwater, especially with the holes already in the tank and how to set it up.

 

Can be as easy as setting a bucket inside the tank. The the drains flow into the bucket and overflow into the tank and having a pump pump the water back to the display