The ride to Amalfi coast is very windy so be sure car sickness is not an issue ( for me it is unless I drive) with all. Second I would reccomend Pompei over Ercolan although you will need more time. You can certainly do all this in 8 hours if you must. Posiatano has alot of up and down walking so be prepared. Ravello is smaller and more managable in a short visit.

Enjoy, David USA