Been in the same research mode . I am coming to the conclusion good and cheap may not be the same unless a temporary solution.
There is a guy on Ebay selling rebuilt pumps with ring diffusers weighted line package systems. One for a shallow small pond was about $200>$300. His feed back was good.
I have found a quality Vertex diffuser is $150>200 so thats a big cost of a higher quality system.
I have found used Gast pumps on ebay reasonable and the rotary vane is not to hard to rebuild if needed.
I have recently learned since it is a new pond I don't need to be in a hurry so I have backed off and doing research and searches for used pumps some of which are called vacuum or septic but do the same thing.
Still researching.