I'd suggest a 2600 gph or greater Intex with sand filter for your size pool. At least 14", if not a 16". There are other pumps and filters that are less expensive, but do check if they come with timers or not. You can't really go wrong with a nice big pump and filter, plus the sand filter is so much easier to deal with as opposed to the cartridge filters. Backwashing is a breeze compared to spraying out those stupid pleats.
Also, I just noticed something while checking out new ones at Amazon... I'd be very wary of the following if it can't be disabled. I believe it should be able to be disabled, but you will want to be 100% sure of this before you buy. Aeration will raise your pH. I think this negative ions thing is bs.
Amazon.com : Intex Krystal Clear Sand Filter Pump for Above Ground Pools, 16-inch, 110-120V with GFCI : Gateway
www.amazon.com