Agree with #9 about Sintra, but will our OP have the time for that? Spending time in Portugal will result in less tim in Spain and a serious loss of stupid time on the roads.
And what about your interests Ian? You tell us nothing about that. You seem to be mainly focussed on cities (Faro and Murcia seemed rather random choices) while there are so many smaller gems along the way where you will driving by and which are more accessible with a rental car. Those smaller places can often be much more rewarding because they are so easy to drive into without much hassle and it is easier to get a feel of them with little time. I rather see places decently in a more geographically compact area than just hitting the surface in more places in a very superficial way, everywhere and also when I travel outside of my continent.
I recommend you to read a guidebook to Spain or eg Andalucia alone :)
