Your rental car in Portugal is covered by comprehensive insurance. If you want additional insurance with more coverage, you can include it with your booking, at a low price.
If you want to use a navigation system, you can arrange it locally or online. When making your reservation, choose the options you want, and the Avis staff will make sure they are reserved when you pick up your rental car. And if you indicate this in advance, you can also pick up your car at the airport and easily return it at another branch.