if you are going to get corals, then you want a little more than one powerhead, probably.
Ina reef tank, you want alot of movement, between 10 and 20 times your tank volue per hour of circulation. In a reef, I really reccomend going high, like 20.
In a fish only, some say 4x is fine, but I personally will not go below 10x in any tank. It serves a great jsutice, in all the areas mentioned, esp in the biological filtration and helps with denitrification. Gotta have circulation.
Yes, youcould possibly get on ph in a smaller tank, but to achieve those numbers, it would require a big one, which would be quite powerful in one area in general. I like multiple sources it calms things alot, while fiving you your targeted movement.