Pond pump questions

I'm looking into building a pond and the pumps have me confused. I'm either going to buy a 360 gallon stock tank or use part of a liner from an above ground pool we are takeing down. If I use the liner I don't know how big it will end up being, as big as I have room for I guess.
So I have be researching and looking at pond pumps and don't have a clue. I plan on making my own filter and just need a pump to suck the water out the pond and into the filter. From what I have seen marketed as pond pumps are like $80 and that seems extreme to me. I also have found fountain pumps which are 1/3 to 1/4 of the price of the pond pumps. What's the differance? I want to get the biggest of everything I can afford. Can anyone shed some light on this for me?