add a water fall the taller the better cooling it will offer. also build it so no matter what time of day it is it that a small portion of the deep end is in the shade constantly. and as for cheap filtration run a giant diy wet-dry. make it using a plastic 55 gallon drum and use pvc shavings for media they sell it at home depot and lowes, but you might be able to talk them into giving it to you free. just wash it good. and keep the filter in the shade(maybe even in a shed to hide it. if you run the piping under ground that will also aid in cooling (and keeping it warm in winter if you run them from the deep end 4-5 fet under ground. also the deeper you make the pond the more stable the temps will be.
there is an asian guy named kenny that works at the ocean floor(hate the store but kenny is a great guy with some crazy connections over in asia) and he is the fresh water god. he has a pond here in arizona and he keep and breed arowana's in his pond

so talk to him and Im sure he'll point you in the right direction and listen to every single thing he tells you he knows his ****