shimano xc30 mtb spd shoes for CycloCross?
If you throw some studs on those XC 30s ( don't skip that step ) you'll have plenty of grip and all around performance. Unless you travel into some higher price ranges where the really aggressive tread patterns usually live, you'll be hard pressed to find too much of a difference between MTB shoes and cross shoes.

If you end up in love with racing cross, you'll start to get a good handle on which equipment works best for you.