When you buy a pump, the main thing you need to consider is how much water it will be required to move. Whatever your tank size is, you should plan on moving something close to the entire amount of water through the system every hour.
I based my pump size on the maximum capacity of my tank, which is 1,600 liters. It's a "Little Giant" brand pump, made in USA (I didn't know they still made stuff there). It has a 500-gallon -per-hour flow capacity at 1' head. That translates into over 1,800 liters per hour at 1' and enough to move all my water every hour at the height I plan on pumping to. Here are some pics: