Your pond is just under 375 g so there are several ways you can go, I like a DYI filter set up made by dropping a submersble pump into the pond and pushing the water through tubing at the bottom of a water trough, it bubbles up through 6-8" of gravel, the trough is then topped off with water plants like elodea and water lettuce or water hyacynth, it is then fed across a rippled splash tray back into the pond.
This way actually cost as much as many standard commercial filters but I like the sound and the plants, I also like using the plant trough for guppies. For your pondputting this set up at one end or side would add 26". It would consist of a 60$ pump (1250gph) and a 100 gallon stock trough, about 200 lbs of gravel, 80 or so floating plants, several bunches oh elodea, and some odds and ends. they work very well but do need a mesh pre screen at the pump.