The metal frame should hold your top flat. The screws shouldn't be a problem in mdf, because, as James pointed out, they are only keeping the top from sliding around. If you should strip a screw hole out they are easy to fix by gluing flat tootpicks in the hole and then re-driving the screw in. Probably stronger than original that way.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.