What road tire should I buy for my new cyclocross
It really depends on how much your budget is. Usually tires with lower rolling resistance comes a bit, if not alot, more expensive. Sk if you are just going for a pair that's cheap. I think it's best if you go for a 700x30. Since this one has the least amount of tire against the road.

I'm also trying to figure out why you don't want to invest in another set of wheels for commuting? Which in that case would mean you can try a road bike tire. A 25 or 28 road tire wouldn't be bad for a cyclocross bike, and I guess it is also ideal for local urban commute.

Again, it is up to you.