Tarheel Tractors, on Ebay, is showing New Holland TC30's (gear models) starting at $10,800 with your choice of tires, brand new with full warrenties. They list them with Great Bend FELs for $13,700. New TC29's with the NH 7308 FELs are listed at $15,700. Corriher's, also on Ebay, is showing brand new New Holland tractors at similar prices. They also have a TC24DA (hydrostat transmission, 24hp) for $11,799. Corriher's 60" rotary cutter is priced at $625 new. That leaves a lot of money remaining for more brand new implements, even after you pay for shipping costs to get the tractor & rotary cutter.
I'd say that the Challenger must be one heck of a great tractor for you to be willing to pay $18,000 for a used unit with a used 4' rotary cutter.