Depends on what you have running the tank to consider how much electricity it will use. Although thats a little tricker to figure out cuz you need to figure out how many watts each piece of equipment uses.
75gal you are going to want a 300watt heater at least. Generally you want at least three watts per gallon for a heater. It might be better to split it up into two 150watt heaters though. That way if one heater fails you will still have a second heater to keep the tank comfortable. Also you have a better chance of having a more consistent temperature throughout the tank.
Filtration again depends on what you like. I personally use H.O.B filters in conjunction with canister filters on my tank. Again, better to have more than one filter on the tank in case one stops working, plus its MUCH better to over filter your tank then to have 'just enough'.
Good filter brands would be Fluval, Ehiem, Marineland, AquaClear, and Rena.
African catfish is SUPER broad, could you narrow it down? Depending on what species it is, there is a great potential for it getting eaten by the oscar. Same goes with the polypterus.
Get the tank up and running, and cycled, then look into investing in some other fish.
