Just did this a couple of months ago for my new "Paulk style" workbench top. I used 3/4" birch ply for the top, screwed on for when I want to replace it. I also used the Parf system for drilling my holes in the top, which worked out great. I left the bottom (glued on) as a solid surface for an assembly table. One thing which I realized about 1/3 of the way through drilling all the holes in the top is the horrible blow-out when the drill went through the last layer of the ply -- I started using a clamped-on backer board, but still got mediocre results.

On my next top, I'm definitely going to drill the holes before attaching it, and use a proper backing board. MDF is probably more forgiving, since the ply I used just looks terrible when I have the thing flipped over in assembly table mode (even though I am the only one seeing it, and you have to look inside at an angle, it still bothers me.) With one sheet each of 3/4" birch ply for the top & bottom (and 1/2" for the webbing), the table is still light enough for me to pick up, maneuver around, and flip without too much difficulty. I'm not sure if the added weight of MDF would make it unwieldy. The top is flat enough for my purposes (I can't see a light underneath a 4' level, but it's not surface table flat.)

Hope you have much success!