Actually, those little preformed ponds last quite a while as long as they are set in the ground. It really doesn't seem practical to build a pond (which is what you seem to be describing) and then put a plastic pond in it. Particularly so since it would be almost impossible to create a good fit for the preformed in all that surrounding structure.

I have one that has been in the ground since 2004 and it is fine. All you do is dig a hole, put it in, and backfill around the edges with sand. I don't have a filter or pump on mine- I only have a few plants and some mosquito fish, so cleaning it out once a year is sufficient.

lucille,
I jsut bought the little pond at HD today. it is 35 gallons.
I wanted a small water garden out front in front of my living room window.
They seem pretty sturdy to me, and since hose said he had one since 2004 and it is still fine, I am jsut going to dig a hole, put it in and plant all around the outside of it. It should be fine.
Post a pic when you are done.
I jsut don't know when I should do this, it is still cold here.
I don't think putting plants in it now is a good idea.

I am still working on the picket fence, I don't anticipate starting the pond project for a few weeks. I did find a good deal on a pump. Hosenemesis, you sound as if you have a good system with the fish but I anticipate a bunch of birds using my pool since I am going to be putting birdhouses nearby, so I am going to clean it out fairly often. I'm thinking I can just pump out water to a nearby flowerbed when that needs to be done.