I'm with JAC. I had a 250 with my previous TT toy hauler and had to get a 350 for my current 5er. I don't recommend a 250 to anyone thinking of towing, get the 350 right from the get go. My CCC went from 2251 to 3447 on equally optioned trucks with the 6.7. The '17+ 350 gets you a heaver rear suspension, bigger rear axle, and different transmission.
My wife is 5'3" and with the stock cab steps, she can get in and out just fine. The tailgate heights bother me more than the cab heights. If you are thinking Ford, make sure to get the step built into the tailgate. I'm 5'10" and it's a lifesaver.