We need more info. $600 seems high to me for 3 basic keggles and a metal rack, but if there's other equipment included (burners, pumps, etc.) or if they're really tricked out then it might be a good deal. Do any of the keggles have false bottoms, sight glasses, thermowells, whirlpool returns, or other features (besides broken thermos)? What type of valves do they have, and are the fittings welded or weldless? Is the metal rack just a rack, or is it an actual brew stand with mounts for burners, pumps, etc.?
FWIW a basic converted keggle usually sells for ~$100.