I think that Europcar has an office in Potenza, but you might do better to rent from a larger town if you want an automatic. Trying to rent an automatic in Italy is rather like trying to rent a manual in USA.
Driving country roads in Italy is no more difficult than driving country roads in USA.
On some of the lesser-used country roads, they might not be exactly 2-car widths wide so always watch for oncoming traffic around the corners. Some of the these roads aren't lit at night so be careful.
Italian roads are generally better maintained than US roads overall.
There are 2 rental car companies here in PZ, Europcar and Avis. Neither are close to the bus/train station and have limited vehicles and prices are higher than larger cities. Getting an automatic may be a problem.
Where are you coming from? There are numerous rental agencies with more options in Salerno where many people take the train to.
twin - The drive to San Fele is not difficult, I've gone there several times doing genealogy research for Americans. If you follow a GPS it will take you on some minor roads with a lot of curves but if you look at a map you'll see taking the SS93 and then turning off towards San Fele is the easier route.