The door knob hole will make a great place for drills to go through rough work if it's on the front side of the bench. A foam plastic coffee cup will fit in it too, but you already realized that. There's all kinds of possibilities for that hole that we just aren't thinking about yet, but it will be there and very handy when the need arises. For now, just keep your coffee from getting knocked over by using it.
I had a solid red oak door bench top in my last shop, but I left it there when I moved away. I found the door for it in a flea market. It was a brand new 1 7/8" thick 36" wide door and they only wanted $25 for it, but that was 45 years ago. It was so heavy that it almost did me in, just getting it home and then down into my basement, so there was no way that I was going to even try getting it back out of there when I left. I decided that I could find another solid door after I got here and make another bench, but then my shop here ended up being so small that it wasn't worth trying to use a door to make another big workbench. I did take the vises off that old door workbench before I moved here though and brought them with me. I recently saw another new 36" wide 1 3/4 thick door at my local Habitat store for $60, and I was very tempted, but I left it there. This one wasalso smooth faced and it's sides looked like mahogany veneer, so it was probably solid MDF in the center. Still, it would make a great super solid and big workbench, and with a piece of 1/4" MDF to cover on the top to make the surface last, it would be another great bench, if I only had the space for it. My old bench had six 4 X 4 pine construction grade lumber legs and 2 X 4 frame, all lag screwed and glued together. It was as solid a feeling as a block of granite when working on it, and I miss that in my present benches.
To find one, flea markets, Habitat stores, and industrial salvage outlets are my best suggestions for finding one. The one that I saw recently was in the Kannapolis, NC Habitat store, and I think it's still there, but likely too far away for you (didn't check your location).
Charley