A car is essentially required on Maui. The island is large, the public bus service (Maui Bus) covers the main towns but runs infrequently and does not go to Haleakalā or Hāna, and taxis and rideshares are limited outside Kahului and Kihei. Rent a car at the airport; major and independent operators are both well represented, and booking ahead in high season is important.
The Road to Hāna requires a standard car unless you plan to go around the back way (southern route) in which case a 4WD is recommended for the unpaved stretch. The drive up to the Haleakalā summit is on a standard paved road and any car can manage it. Expect some congestion on the Honoapiilani Highway on the west side in rush hour and around popular beach parks on weekends.