Now is fine. We built ours in August and thought nothing much would happen till the spring. It took the frogs less than 24 hours to move in.
I don't know what size you have in mind, but whatever you plant in it be prepared to thin ruthlessly every couple of years. There are irises, reeds and mints and loads of other things you can plant in the pond. For me the crucial thing is the oxygenators that go under the water and the marginals that go round the edge. I am about to take out practically all the plants in my wildlife pond e.g flag irises that self seed and take over.