I'd decided on an oil-and-wax finish. Oil finishes are by no means the toughest. In fact, they're really rather pathetic, so far as protecting the wood goes. But they're easy to apply, and not even the toughest finish will stand up to the abuse that a workbench will suffer, so it's more important that it be easy to repair. Wax is usually used to add a high gloss. On a bench, it's there to keep glue from sticking.
Put the upper panel of MDF on your glue-up surface, bottom side up. Put the bottom panel of MDF on your other surface, bottom side down. (The panel with the holes drilled in it is the bottom panel, and the side that has the your layout diagram on it is the bottom side.) Chuck up in your drill the appropriate driver bit for the screws your using. Make sure you have a freshly-charged battery, and crank the speed down and the torque way down. You don't want to over-tighten the screws, MDF strips easily.
I made a low table out of a couple of step-stools, my hollow-core door, and one of the MDF panels that would eventually form part of my top. I was concerned that any oil that dripped on the door might interfere with its glue adhesion, when I finally get around to the project for which I'd purchased it. The top side of the top sheet of MDF, though, I planned to oil, anyway. (Ditto for the bottom side of the bottom sheet.
The bench I will be describing differs from both of these in a couple of areas, the most significant of which is the top. Allen's top was made from three layers of 3/4" medium density fiberboard (MDF), topped and edged with 1/4" hardboard. Christiana's top was just two layers of 3/4" MDF. My top is two layers of 3/4" MDF edged with 1/2" oak and topped by a 1-1/2" thick edge-glued oak Ikea countertop. My top is more expensive in both time and money than either Christiana's or Allen's. If you're looking to build something fast and cheap, I'd recommend Allen's approach over Christiana's. The hardboard significantly increases the durability of the top.