Will you compete in cyclocross races or are you just looking for a bling bike? Because if you're seriously gonna race a 5k crossbike - especially with a carbonfibre frame, carbonfibre wheels or whatever - you'd better have another 5k in reserve for repairs, spare parts, etc. Don't race what you can't replace.
I second "Frunkin": get yourself one or two bikes with whatever $500-$600 frameset appeals to you, Campagnolo Centaur or Shimano Ultegra/105, Spooky brakes, several wheelsets (36 spokes, Mavic Reflex rims), 12-25 and 13-26 cassettes, tubular tires for mud, dirt, sand, gravel (30, 32 and 34mm), Time pedals and go.