Yes, if you fly to Madrid you can get a direct flight to Fuerteventura. The easiest car rental would be to rent from the airport - as I posted earlier, car rental is cheap. Pluscar are very convenient, you just leave the terminal, go into the car park, and they are waiting for you with your car in a pre-assigned parking place. No excess deposits, no silly insurance scams, no fuel scams, just pay the rental and go on your way. The roads are fairly quiet, and well sign-posted. If you get lost, Google Maps is your friend!