We have several ponds in our garden. I love ponds, almost every year we've had frogs and tadpoles in our ponds. Some had yabbies. I tried to put in native fish but they never survived, finally I caved and put in something I never intended to put in my ponds, goldfish. You'll need something to eat the mozzie larva that will most likely infest your ponds. Freshwater Mussels, if you can get some are also great in a pond, they help filter the water.
We have plenty of aquatic plants in our ponds, this gives the taddies somewhere to hide from the goldfish. We also have the dreaded Mosquito fish in our ponds. In SA this fish is declaired a pest species. We have no idea how they got in there.
I'm not sure where you live so hard to give advice as to what plants ect will grow well. Here ponds are best placed in semi shade because it's like an oven here in summer and the evaporation rate is so high. As far as I know if you want water lillys you need full sun. I also have water Iris in my pond.
One of my ponds is an old bathtub, pretty good, the other one is like what you want to build, a plastic sheet with rocks around the edge. With the plastic liner, make sure you buy the thicker stuff that is actually sold as a pond liner. This is a little more expensive but it is well worth it as using normal black plastic always gets leaks. Make sure you line the hole with sand first as recomeded in all the books, or try our idea (a little weird but it worked) try lining the hole with a thick layer of old newspapers instead of sand.
None of our ponds have filters or water fountains. We've never had problems, our water is always nice n clear, but have seen other ponds look dreadful, green, smelly ect, not sure what we are doing right but I suspect it is the amount of aquatic plants we have.
Good luck with you pond Brett, yes we'd all enjoy looking at pics of your garden.
Whats the most important thing on earth? .....WATER. Without it we have nothing.