Yes the 250r is enough bike and no the 500r is most like not too much bike.
The ninja 250r will do freeway speeds without a problem, it tops out north of 90mph. It easily handles my 6’2″ 220lb frame though the newer model has a minor ergonomic issue for me, it won’t be a problem for someone more average height.
Take the MSF first before deciding what to buy. The class is great and you will be much more comfortable on a bike after the class.
The Ninja 250 felt like a much bigger and more powerful motorcycle than the MSF bikes (TW200, Rebel, Nighthawk) but still light and easy to ride.