Steel wool will take a while. I'd say a wire brush on a grinder or drill, then when it's clean enough scrub it down with coarse kosher salt and some steel wool (just too remove any particles from the wire brush), then rinse. At that point coat in oil and put it in your oven for about an hour.
It's definitely doable to clean it up, but to get it done and ready to cook on in a day might be a stretch... I've seen Lodge griddles at Wal Mart for about $20...