No motorcycle will overheat at 75 mph unless it has a problem with the cooling system- leaking coolant, bad thermostat, bad water pump or oil pump, blocked radiator or cooling fins, etc. Air cooled engines could overheat creeping along in a traffic jam on a hot day, or in a parade, and generally air cooled engines wear out faster, but are lighter and simpler.
The 250cc and 500cc Kawasaki Ninjas are water cooled, and will go 75 mph all day, except for a lot of weight added to the 250 (like a 250 pound + rider) going up a big hill, but you will still probably go up the hills faster than the heaviest trucks with trailers.
The biggest problems with the 250 Ninja is that a passenger is not comfortable, bigger bikes will beat it on a race track with a skilled rider, and if ridden hard all the time (high rpms) the engine often wears out at 60,000 miles. Its 95 mph top speed is fine for the street. Also, many people have too much ego to ride a bike with a narrow back tire like the 250 Ninja has, and the pre-2008 250 Ninja has an older look and fewer tire choices.
See the recommended reading on the right side of this page for more good bike choices for a beginner on the street.