I'm getting ready to buy a new outboard. From the top of the transom to the bottom is 17'. I don't know what length shaft to get...Need help.

That's an odd lenght.   Is it built with a built in setback so the bottom of the hull is actually a few inches below the bottom of the transome?  If so, then you need the 20" shaft (long shaft)

I was recently told that you want your cavitation plate on your motor to be equal to or below (though not too much below) the bottom of the transom, not above it.    The cavitation plate is the part where you would mount a hydrofoil if you had one on your motor.  Hope that helps.  You could take a measurement from the motor's mounting bracket down to the cavitation plate to see if it would be long enough.

Found this diagram,maybe it will help.