Look at the RMS of the speakers and sub and go from there. Try look for two amps, one for the front/middle stage and one for the sub. But it depends on what sort of setup you're after.
I would also look at removing the 6x9s for just some filler, the sub should be producing all the necessary bass. That is of course, if your front stage (front of the car) is decent.
Me personally, I'm not a fan of buying based upon brand name. Don't go buy an amp or sub because it's cheap or bacause "Sony rox", research it and try listen to it in a car not on a board. Speak to the installers, see what they are running and recommend. Spending a few extra weeks and a few hundred more doesn't give you these situations where amps just die.
All lessons though, we are all here to help you through.