I used 2 layers of Medium Density Fiberboard (MDF), glued and screwed together. For support, I used 2x4's underneath to make cleats on the wall and for "stiff back" to keep the table top from sagging. I used 4x4's under each press that are screwed to the floor, and the top of the table is screwed to the top of the 4x4. The "stiff back" is also screwed to the 4x4 legs. I think my bench is about the same width that you have in mind, about 20" or so. You can't have too much space!
I didn't want my bench bowing under any pressure, up or down. I painted the top with exterior house paint.
The oak planks you have sound great! Just make sure you screw them to some sort of frame underneath or even those mighty oak planks will sag or split. I wouldn't leave too much "free span" between supports that go down to the floor. You can exert A LOT of pressure up and down with a reloading press sometimes. Like I said, you can't have too much space, but make sure you spend a few $$ on braces underneath so it won't be moving around, and don't scrimp on the screws and glue.
It's not that any old table top wouldn't do, but since you're building it, you might as well as build it as if you expected elephants to dance on it!