I recommend renting from Europcar. It takes less than two hours to reach Evora from Lisbon (and you will have the flexibility to visit the Os Almendras site, which is not reachable by bus). Parking is availabe either outside of the city walls or next to the theater.
Roads are incredibly smooth, but expensive - about E10 in tolls from Lisbon. Gasoline is also incredibly expensive, which is why most people drive here diesel-powered vehicles. But the flexibility in my opinion is a great argument in favor of renting. Once here, you don't really need a car to see the city, since it is compact and walkable.
If you travelling by car from Lisbon, it is very easy and fast to arrive in Évora – just take the A6 highway and soon you will arrive. But, be sure to park your car in one of the authorised areas inside the walls (no much parking places though). Probably the best alternative is to park in one of the parks just outside the city’s walls, by the gates. These parks have lots of places to park your car.