Rental car offices
With Sunnycars you get a total package of a rental car with insurance. It's a little more expensive but might be worth it to drive worryfree!
Rentalcars gives you a good overview of all the rental car offices in the area where you would like to pick the car up. You can add their car insurance or rent a car without an insurance and use a different insurance.
Rental car insurance
If you don't want to pay to much for your rental car insurance there are options to get an insurance from another company then where you rent your car.
Airlines that fly to Porto
Ryanair fly from most big cities to Porto. Check their website to see if they fly from your city!
Transavia flies from Amsterdam to Porto
Cheaptickets is a flights search engine that shows you the cheapest ways to get from one city to another.
Skyscanner works the same as cheaptickets but sometime they come up with different prices, so check them both!