It's easy to buy a car but not so easy to sell.
What do you intend doing with the car after 6 months?
I would drive over on ferry from Portsmouth to Bilbao and then it's only a 10hr drive to Costa del Sol and that way you can bring more things with you as you will be staying for 6 months.
The Spanish have a habit of always blowing their horn at UK reg cars if you don't take off when lights change lile a F1 driver but just ignore them. Switch over to ESP plates and that all stops - amazing!
Other than that I would try get a good long term car rental deal from say ATESA (citroen).
Property rental is also easy but I do recommend you inspect property before renting etc. and check absolutely everything is working!!
Do you intend staying in one place for 6 months or going different places. Spain is a big coutry and lots of variation from coast to inland & north to south. I might be inclined to do a month here and a month there etc
Becareful with rental contracts and deposits - lots of sharp practice out here - trust no one!