You can find everything you need on Ebay or online stores. An LFS
is giong to want a lot of money for those items. When you calculate your flow rate remember that your pump needs to move 300gph at the hight of the back of your tank. So a 300gph pump would not accomplish what you want. You will probably need a 500 or even a 600gph pump if you are pumping up 4' to the top of your tank.
I have a dual sump set up. I have a 20g tall that is my fuge. It has LR LR
rubble about 4" of sand and a ton of Macro in it. I have a dual overflow (external) one side of it is plumbed to the sump and the other side is plumbed into my skimmer. Both the Fuge and skimmer dump into a 10gal tank that has my return pumps in it along with more LR
I'm not sure how large your HOB
is but using it as a QT doesn't sound like a great idea, unless it is very large. A 10gal tank from Wal-mart is about $10.