starting over

I have a backyard concrete pond that measures 12'W x 15'L x 4'D. Until
recently it has been FULL of lilies and a very dense root system,
anchored by substantial sediment. No fish. It has never been filtered
but never had an algae problem because of the amount of vegitation
surface coverage. This year, however we had an infestation of
caterpillar type worms that chewed all the leaves, leaving only stems
and a very ugly pond. I decided to drain and clean it and start over.
This time I'd like to have a few Koi and a more attractive pond. I am
planning on putting about four inches of gravel in the bottom and
replanting just a few of the lilies. My question is how should this
thing be filtered this time? What equipment, etc. for max clarity and
least maintenance