You'll want a pump that does at least 150 gallons an hour and I would reccomend a primarily biological filter with foam pads for mechanical filtration. If you're able to gt your hands on some bulk heads, then a garbage can or plastic bin would be very easy to convert but I am not sure how to cut down the cost for the biological media (bioballs or whatever you choose). The bravity fed biological filters made by Tetra work very well on ponds.

Matt