Solid core doors make good bench tops, but it'll probably be a particle board (sawdust) core & you'll need to seal it somehow. You'd probably wanna put the cut side to the wall. I've seen some people rout out the particle board 3/4" deep on that side & install a solid wood filler glued up. That'll help. Of course it goes without saying you'll probably end up cutting the lock side of the door unless you need the lock hole for cords & hoses & such. Before you cut the backside to dimension, trim off the hinge mortises & sand it up. Be careful here, most doors have a narrow stile on each side so just trim off the mortise + 1 sawcut.