Benny I think 1 1/2" slate would make a nice table surface!
On the other hand you might vacuum bag two or three layers of fiberglass on the surface of mdf and then sufrace it with your cnc to get it level and smooth. I think the melamine will dent also. If you use mdf or high density you need to be sure and completely seal and paint all exposed surfaces or your work will be in vain.
Mike