Can you give a bit more information about the pump? The website doesn't give any information until after you've "signed up". If it's "24VDC", you can wire your panels in a 2S2P configuration, feeding ~24VDC (under load) at maybe 8-9amps.
There are battery-less pump controllers that let you wire the panels directly to the controller without batteries at all. If finances are tight, then don't get sealed lead-acid. Regular lead-acid would cost half as much, and last twice as long.
A lot of solar pumps benefit from higher voltage, and 400W is not much. How deep is your water? I've seen Grunfos DC solar pumps with voltages as high as 300VDC. They even have a pump that can run at either 240VAC or 240VDC.
Your best bet for higher voltages though are grid-tie panels, which are far cheaper per watt than 12V panels.
