Well, I shouldn't be the one saying this, but welcome to reefing! Your new 75gal. aquarium is going to be reef as well as fish. You need to decide what kind of reef compatable fish you are going to want, as well as what corals you want to keep. This all has to do with what kind of lights and food you have to buy. The second part of your question or confusion is that for your aquarium, You will have to purchase a "siphon overflow box" or "CPR Backpack". This device will hang on the back of your aquarium. You have to start the siphon manually. It will only siphon water out of your aquarium when the pump is turned on. Due to the design, if your pump turns off, the siphon stops, but does not break. This means that when the pump is turned back on, the water is pushed back into the aquarium and resiphoned. If the siphon breaks, your pump will push water into the aquarium, overflowing it as well as running your sump dry and ultimately burning out your pump.
When choosing a pump, you need to make sure that the pump can push 10x the volume of your tank per hour at 5 foot of head pressure. for a 75gal. aquarium, I would recommend getting a pump called a "mag 9.5" This pump pushes 950gallons at 0 head. I think that for a 75gal. aquarium, a mag 12 would be overkill. (I have a mag 12 on my 90gal.)
As for the plumbing, many people have said that vinyl tubing is the best. I say that it is the best for starters as well. Just cut your tubing, stick it on the return of the pump, and buy a directional U-tube and stick the other end to that. If you know a little about plumbing, I say get a 3/4" PVC pipe, some PVC primer and cement, compatable ball valve and check valve and some elbows. Make sure you also have a union if you want to get your pump out and clean it occasionally.
As for what type of refugium/sump: I say that you would need at a minimum of a 20gal. or you could just buy a rubbermaid that will fit underneath your tank. A refugium is for growing macroalgae, so make sure you have enough money to buy a light for it as well. Happy reefing!