Making a filter

It's not exactly for bettas, but it does come under the habit/accessories bit.
I'm building a filter for my pond in an attempt to save myself $300 (especially since I have to pay roughly that much for the pump anyway).

My plan is to get a 5 gallon bucket. I will cut an inflow hole at the top and an outflow hole near the bottom. I will put a brick at the bottom of the bucket, covered by a grate, to create a space for the water.
Over the grate I will place a fat fat layer of sponge, then a layer of ceramic noodles, then a layer of filter wool.

The pond is 500 litres and will house 3 goldfish (2 comets and a shubunkin). At first I will just have a couple of silk plants, but I hope to introduce some real aquatic ones soon after.

I have some questions:

1) How many litres per hour should the pump move? 5000 (10x the capacity of the pond? Less? More?)
2) What sort of aquatic plants are good for a pond? It will have a large river stone substrate, but the pond has ledges on which I can put pots. I may be able to do some semi-aquatics, too, on the higher ledges, like violet (which I love).
3) Goldfish people, is my stocking ok? I love shubunkins and I really want one, but if it would be better to get three comets I will. I want the fish to be happy.

Íf anyone can answer any of these questions I'll be very grateful. I'll keep y'all updated on my project!