look into getting a skimmer box especially if you have a lot of trees around your area, the pond will stay much cleaner....they do make some mini ones for small pond applications...you can make your own waterfall basin to save some money.

external pressure filters are ok but can be a pain to flush out all the time, but make it easy to incorporate a built in UV sterilizer...fishmate brand is usually pretty reasonable, you will have to buy a separate pump so mag drives work well for small ponds.

once you narrow down what you want send me a pm and I can get you some rough pricing on pond equipment....