My pond is very similiar to yours (about half acre, a little deeper though around 10').
I found a used Gast Rotary Vane Air Pump on Ebay $80 delivered a couple years ago. It was used in a science lab at some university.
I also bought an Intermatic Timer Switch on there (dont remember how much I got it for, maybe 20?)
Use some garden hose to get the air to the pond and some of that Soaker hose weighted down in the bottom for the diffuser.
Thats about as "Cheap" as it gets for a 4.5 cfm Rotary vane system, but thats what I wanted.
I have slowly upgraded as I had the means. Bought a Membrane diffuser from Bill Cody. Works tons better than the soaker hose. Just a little higher up front cost. I could probably put some pictures up tomorrow if it will help. Also added a nice pressure gauge and PRV in case it freezes. Might not need that by you.
