We are going to take a bus from Tavira in the Algarve region and cross into Spain. We would like to then rent a car and drive to Arcos de la Frontera and Ronda. What town just over the border in Spain would this be easily done?
There are Avis, Budget, National, Alamo, and Enterprise locations in Huelva near or at the the train station. That may be your best bet, but Huelva is 30 miles from the border. From Huelva you could also take the train to Sevilla and pick up a car there.
Is there are bus options to Sevilla, they may be better.