Koi pond owner here...
In ponds I let the plants do all the filtering, and from the sounds of it, that's what you want, too. You don't need a traditional filter like you would in aquariums; you really just need something to keep the water moving. A simple bubbler or small fountain works well for this.
As for the summer, enough floating plants will shade the pond. Also try to position it in the shade if possible. Another thought for the summer is rain storms; you will want to look into some sort of overflow device, a spout to let the water out without hurting the plants or the fish. This is going to depend on the kind of pond you get.
For the winter, this is going to depend on your location. If it gets really cold, you might need to move the pond to your garage or other indoor place during the winter. If it doesn't get too cold you could bury it in the ground (which provides some insulation) and get a small pond heater. Farm animal water trough heaters also work for this. Overwintering in a tank inside is always an option, too.
I've always thought these projects to be neat little things. Costs a looooot less than a real pond, but you still get the same aesthetics and the lovely sound of running water.