Driving our car to Spain and the 6 month rule

We are driving our car to Sucina in December and going to initially rent a property for a year or possibly longer in view to buying if we find its for us. The problem is the car, I have just am confused about what to do because of the 6 month rule. We have an old car which it's mot runs out in April 2019 are we the allows as non residents to buy a second hand Spanish car?  What I don't understand is they say you are classed as a permanent resident if you stay longer tha