I have had a pond for quite a while. You have basically two options for the main pond - a hard liner or tub or whatever or a flexible liner. Mine is a preformed that holds approx 180 gallons. Preformed are nice because they are usually a bit sturdier. The down side is that you don't have the flexibility of designing your own shape around the space you have. With the liner, you would need to figure out the edging or some other way to secure the top edge of the liner. My sister used a liner for her pond and used decorative block to hold up the edge.
You will also need to decide if you want substrate or not. My pond is
bare bottom - it makes it easier to rake the debris out of the bottom. My sister's pond has large river rock in the bottom to anchor her lilies. I just put the lily root in a plastic mesh bag with a couple of rocks to weight it down.
As for size, I would go as large as you have room for. It is better to start with a larger size than have to upgrade later. I wish I had more room to make a larger pond. I have already upped the size of mine as much as I can in the space I have available. (and I still wish it were bigger!)
The regular feeder goldfish are fine. Just be very picky about the ones you buy. Sit and watch them for at least 30 minutes and insist on the ones YOU choose. Don't settle for the ones the employees catch for you unless they are the ones you chose. When I was buying mine, I actually had an employee swipe the net through the group and dump it in the box saying "here you go". I said NO. I want THAT one and THAT one, not these ones... You may have to be a bit insistent, but it will pay off. All of my goldfish started out as feeder fish from WalMart and PetsMart. I did lose a few of them, but mostly they have survived. If you really watch them, and make sure they have no signs of disease, you should be fine. Don't pick them just because you like the coloring or the tail or the cute face - you really need to be picky healthwise. When you bring them home, quarantine them for at least a couple of days. I usually dosed with melafix when I added new fish - just as a precaution. The main trouble I had was watching for
ICH. I always keep some ich attack on hand so I can dose as soon as I see it.
Other Fish: I would not recommend a common pleco unless you plan to bring it inside during the winter. I have two sailfin plecos in my pond. They do fine in the summer, but I bring them in the house for the winter. They do not do well when the temp stays below 72* farenheit. You may want to throw in some white cloud mountain minnows. I have not found any around here, so I don't have any experience with them. I do have mosquito fish in my pond. They may do fine in yours as long as they get some full sun part of the day. If they are completely shaded all day long, they will not survive. I am experimenting with American Flag Fish this summer and so far they seem to be doing fine. They feed on algae, as well as on small insect larvae so they would be good scavenger/clean up fish. They are also quite pretty. Oh, and I also have a chinese algae eater in my pond that comes in during the winter.
In the colder areas, you would need to have your pond deep enough that most of it will stay unfrozen. The deeper water will also maintain a more stable temperature. The goldfish will be fine year round, just make sure that you keep a small area open for the gas exchange. Otherwise your fish could die from oxygen starvation.
Plants: Google water plants native to your area so you can get an idea of what plants would do well in your area. Pond lilies, water hyacinth, and water lettuce are all good floating plants that will help to provide cover for your fish so they can hide from predators. They will also help to shade your pond so your fish don't get sunburned (yes it is possible for a fish to get sunburned - mine did). I haven't had any luck with underwater plants, my fish eat them all. Water hyacinth and water lettuce are illegal in my area, so I just have pond lilies.
Filter: Don't skimp on your filter. A good external pressurized filter with a uv bulb is a good investment. Figure the gallons in your pond, then add extra for a waterfall if you want. You should also add extra if you want more fish, especially since goldfish are a very dirty fish. Then get a filter that can handle more than the volume of your pond. For example: 200 gallon pond + 20 gallons for waterfall and stream + 80 for lots of goldies = 300 gallons so get a filter that is rated for at least 300 gallons, preferably 400-500 gallons.
Pump, same basic rules as the filter. Get one that will handle more than the size of your pond. It won't have to work as hard, and you will be able to add additional water features with out having to change the pump. It is also a good idea to have a back up pump in case something happens to your main pump.
My pond is set up so the pump sits in the bottom of the pond and pumps water to the external pressure filter which then returns to the pond via two waterfalls. It does not freeze too much here, so I don't need to worry about the tubing freezing. You may need to bury your tubing or wrap it with insulation or something to protect it from freezing in the winter. Having a waterfall should help to keep the pond from freezing over. The waterfall will agitate the water where it falls into the pond and moving water does not freeze as easily as still water.
You should also have:
A pond thermometer to monitor the temperature of the pond.
A
skimmer net to get the leaves and debris out of your pond (especially after a wind storm).
A
test kit - yes, even though it is not an aquarium, you should still test for ammonia and such.
Two (yes two) fish nets - one with a fine mesh for the smaller fish, and a larger one for your goldfish as they grow. For my large net, I got the hex pond net from Drs Foster & Smith. It is not a very deep net, but it allows me to scoop under the fish and hold them just under the water, in the net, so I can inspect any that may look injured or sick.
Melafix - a good all around medication - wonderful for injuries
Ich Attack - Ich is common with feeder goldfish, so it is a good idea to have some on hand when you add new fish.
Food - I use a floating pellet. It stays on top of the water longer so the fish have a chance to finish it all - if there is left over, you can use your net to fish it out and dry it to use the next day. This helps to keep the pond that much cleaner. No rotting food in the bottom of the pond. The goldfish will also eat the algae that grows on the sides of the pond, and will nibble at the plants you put in. I usually have enough algae/moss growing in my pond that I don't have to feed my goldfish that often. This year, my two plecos are much bigger and eating more, and the flagfish are moss/algae eaters also, so I am having to feed more often. That's okay though, I kind of like it when the goldies swim through my fingers and nibble on the back of my hand when I feed them.
Well, I think I covered most of the basics. If you have any other questions, ask away. Someone here is bound to have the answers. Oh, and as for doing a pond as cheap as possible - it is better to spend a little more at the beginning to start right than to end up spending more later to try to fix things. Figure out exactly what you want the pond to be, plan it out, allow for expansion, then get what you need to make it happen. The only money I have put into my pond for last several years has been for food (and the electricity to run the pump/filter, and the water to fill it up when it evaporates).
So, here are some pics of my pond and fish - I put my fish in a 2.5 gallon to get pics of them from the side since my camera is not waterproof...
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