Seat post clamp racks won't support panniers. I'd mount a standard rack in any case, you just have to use P-clamps.
I wouldn't skimp a lot on a rack. You can move it to a new bike later, so don't buy a total piece of junk.

I kind of like the rack's rear support to be squared off like this one

My rack has a straight rear support, and twice I've had my panniers front clip come loose, and the bag rotated into the spokes of the rear tire. A rack like this wouldn't let that happen. And this rack is only $15.