Ok, since I'm not a damn business and I can buy from a distributor who has what I want in stock on their shelves I decided to go with the mdf. I am going to seal the edges with all-purpose putty bondo and paint it. I really wanted to use that Extira product but as you may be able to tell I wasn't able to buy it today and since I want to start on this over the weekend I went with what I could get. I think it will work out fine as I'm only using the mdf for the sides and front. I'll use a plywood top, I'm considering using a 1/8" black piece of acrylic to put over the plywood top. I think it would look nice and make for easy cleanup. If for any reason I screw up the mdf I'm am going to find a lumber yard to special order me the Extira product. I've read some really good things on it. It's used a lot in outdoor sign making. I understand everyone's concerns with mdf and water, but I think if finished correctly (by sealing the edges) it will work just fine.
Nate, as far as the finishing goes...do you have any other advice other than what you said above? Skim it on and then sand it down?
Thanks everyone.