you can use landscape bricks or wooden 4x4's. definitely get a flexible liner. don't skimp and do a PVC liner, got 45 ml EMPD liner.
Depending on the filtration and size is how you determine what fish you want. over 1,000 gallons with enough filtration is suitable for koi. anything below that is okay for goldfish, sarasa, and shubunkins.
there's only one way to build a low maitnence pond, and building it itself is more work, and a little bit mroe expensive, but in the long run, you save money on properly fitted pumps, filters, your fish are healthy, so they don't die and you but new ones, and you save time by not cleaning the filters as much.
it always bugs me when people put these skimpy pumps on the bottom of the pond with a foam filter pad, and they clog easily, and they get turned off of the hobby.
take my advise, spend the extra $200 now, and save time and your fish' lives later.
Depending on what you want as far as size, I can hook you up with absolutely everything you will need except the water and bricks/wood. everything from the liner, to pump, to filter, to consultation, and yes, even some aquatic life.
a 6x15x3' pond is over 2,000 gallons, and is more than enough for some koi. no more than 10 though.
-Eric