...gallons) and how do you install it?

Pumps are pretty much up to you, based on price. For a 1000 gallon pond (I'm guessing about 5'X7', 2' deep) I'd probably go with something between 900 and 1200 gph. Those can range from about $90 to $300, depending on what company you buy from and how much electricity you want to burn. I've had a few in that range and always had good luck with the Supreme mag drives. I run a 1900 gph Pondmaster in my bigger pond (~1500 gallon) now that consumes about 300 watts of power.I run it through a Cyprio UV sterilizer into a waterfall filter. I've tried other filters that worked OK, but this one is definitely the way to go. It gives me about 2 feet of lift to use in the waterfall, I only have to clean it out once a year, and you can plant stuff in the top of it (I have cattails and something that looks like arugula in it this year).I've had real good luck with AZPond supply (if you're in the USA). Their prices are reasonable and they deliver promptly.Forgot the installation part - Be sure to use a GFCI circuit to plug into. Avoiding electrocution is always a good way to go. I plumb mine with 1 1/2" ABS pipe. It's black so it hides well. My pump has plenty of capacity for my pond, so I'm able to put a T in to run a little spitting fish on one end of the pond opposite the waterfall. The UV sterilizer and the waterfall filter all have 1 1/2" fittings, so that made life easy. You can get fancy if your pond is not in yet and build the plumbing in. I'm not that smart, so mine has to come out of the pond next to the waterfall where I hide it with plants and rocks. I also don't have a "hard" waterfall where everything is cemented in. I simply put another piece of pond liner down on the fill dirt that slopes into the pond. I just piled up big rocks to create the waterfall and use the filter to spill onto the top. Any water that spills through the rocks is caught by the liner underneath and just flows back into the pond. It's not a fancy as building it all in, but it allows me to change things up much easier when I get tired of it.