IMO, you should skip the rubbermaid sump. Over time the plastic will become brittle and will crack and you will get leaks. You can make yourself a plexiglass box or even use a cheap 10 gallon glass tank. I bought one for mine and it only cost me $10 new from the fish store.
I have a 72 gallon tank with a 10 gallon sump and a 10 gallon refugium. Something close to this size should be plenty for you. The sump size is not very critical as long as it is big enough to house the pumps and skimmer that you are using.
A power head will not be good enough. Power heads are designed only for water flow. They are not designed to deliver "head pressure". This is the pressure the water creates while the pump is trying to pump it uphill to your tank. System pumps are designed to maintain good waterflow even with having to pump the water uphill. You will drastically reduce the flow rate of a powerhead if you add head pressure to it so you should only use the Mag pump (good pump).
For pump size, you should have a system that circulates water at about 15 times per hour. That means that your 75 gallon tank should have pumps that deliver 1125 gph minimum. This includes any power heads that you use in the tank to push water around. So add up your system pump and all your power heads and keep it above 1125 gph.
When you build your sump, make sure you use a foam or floss prefilter to clean the water before it enters the sump and wet/dry. Rinse the prefilters out as needed and your system will stay much cleaner and you will not have any buildup with nitrates that some people attribute to wet/dry systems.
As for the bioballs, surface area is key. I would use the smallest, most porous bioballs avaliable. Some people even use ceramic pellets instead of plastic bioballs. Surface area is key.
