Well, actually, I'd probably assume the quilt will be more for looks than actual usage if the person overheats with just a sheet. So, I'd just go with a thin batting available at the store closest to you.
(My husband gets hot and always throws off extra blankets...but likes looking at them on the bed.)
I personally like the feel of an extra layer in the middle as it smooths out the bumps from the seam allowances.