perfectly fine idea. But as mentioned, there are many cross bikes out there with various gearing, frame geometry, all kinds of wheelsets, some without rack mount spots, etc, etc, etc.

20-30lbs is nothing on a rear rack, especially at your weight, on a more relaxed cross bike such as a Tricross. Might not want to jump off curbs, but then that goes for any road bike, and is entirely avoidable.