| I, too, would suggest you wait to dig your hole until you have everything, or at least the following:
liner
pump
filter
It is usually a good idea to raise the edge somewhat. It will keep ground runoff from running into your pond and washing in dirt, leaves, fertilizer, pesticides, etc. You don't have to have it very high, just as long as it is high enough to prevent water from running in. It is also kind of nice to have at least part of the edge be a wall to sit on. That way, you can sit and enjoy your fish.
As for the pump and filter, bigger is usually better (especially with the filter), so you will need to decide about how big your pond is going to be. Once you know that, get the largest capacity filter you can afford and the pump that will run it. I like to get a larger capacity pump than is needed for the filter just because it is going to get clogged with leaves and stuff in your pond. If it is a more powerful pump, than it will still be able to pump the recommended amount of water through the filter until you can clean the pump intake. I have an in-pond pump and an external filter. If I could figure out a way to do it, I would like to have it all be external with a set up similar to a swimming pool. You know, where the water overflows into the pump chamber... That would help to keep the pump from getting clogged by leaves since they would be caught in the basket. | 
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