I'd make a bunch of ribs, notched around your screw, running across the table. The more the better. And use 2 pieces of 1/2" MDF laminated together. 4 pcs of 1/4" would be even stiffer. No matter what you use, it'll sag without support.
You should get it as level as possible by shimming, then surface it with the router. I've got a 2-1/2" slot cutter that came from an auction, that surfaces my 30x45 table in about 5 minutes. Amana make a nice surfacing bit, but it's around $200 or more. Her-Saf has a 2" for about $150. Cheapest is to by a 1-1/2" carbide tipped straight bit. About $25