Aquriums have been around a very long time. Use to be all filters were air powered. Even hang on backs used air to push the water up the tube to the filter then it would just overflow back into tank. Under gravel filters too used to be only air powered having the airstones at the bottom of the tube drawing water through the sand then up the tube. Air flow in water is a more powerful than people think. Then came powerheads people attached to these filters making them more efficient. The action of water falling into the tank adds enough oxygen. Proof is in the millions of people keeping fish like this for years. Would an airstone be beneficial? Of course. An air stone or bar properly placed in a tank will create great additional flow as well as dissolved oxygen without the need of a Powerhead.
You're running an undrilled system with out a sump? No HOB skimmer? If you run an HOB skimmer that will provide more than enough oxygen to your system. If I were you and this is how I intended to do my system with HOBs, I'd do one filter and a skimmer.
Now for water flow. A 70 g is a pretty big tank. You don't mention or have pictures of rock work or sand or mention of corals. Your rock work will deflect flow cascading from the rear to the front of your tank making things and detritus settle in front of it where flow is low. You'll want something to create a sweeping flow across the front of the rockscape to kid the up to make it to the filter be it a Powerhead or a long airstone in the rear making water flow roll the water up over the top and down the front sweeping this settlement back into the flow to be filtered. Whole purpose of water flow, letting nothing settle and getting it to the filter.
Obviously you've been looking at setups like this on the web or you wouldn't think of doing it this way. Totally acceptable and part of the hobby. Using different methods of achieving desired results. If you see others having success this way and that's how you want to go. Don't let others decide a different path for you. Because there'll be those that tell you it won't work and you need to spend thousands of dollars to have a good set up.
Remember, it's not so much of keeping the inhabitants as it is keeping the habitat they live in.