I have a 180 gallon and I have six 950 g/hour power heads, with a 1000 g/hour return on my refugium-sump. this would equate to about 6,700 gallons of water movement an hour and the tank works fine with corals (large polyps and small), as well as the softies. My power heads are on timers and do not run continuously so there is an ebb flow in the tank as well. This works well for my set-up and rock arrangement. I didn't buy all power heads at once and only added more as I purchased hard corals, I think I had two for the fish only tank, then four for the softies and then went to six when I started adding the hard corals.
I think you can be somewhat more practical if you start slowly and work your way into corals. cost is a consideration, also remember that power heads add heat and CO2 to your tank as well. So you need to have a good protein skimmer, don't skim on the skimmer. In my 180 I have a skimmer rated for a 380 gallon tank.
Aside from the sand and rock, the skimmer and lighting will be the next two most important things on your list if you are going to house corals. Power heads can be added, I would start with smaller ones so that you can place them in ideal locations to get the most turbulent water movement. Movement is what the corals need more than velocity.