Hi DJ, the build-up kit is required because it allows the countertop to sit flush on the cabinetry, allowing the necessary clearance for drawers below. Thank you!
No, all you need is scrap blocks of 3/4 inch plywood. I think you just put these in a couple of locations front and back. I've seen photos of people using several strips of plywood a couple of inches wide from front to back on the top of the cabinet instead.
On each corner of every cabinet
Some type of particle board.