Two advices : 1) don't come with a car (we did and were lucky to have Marcello kindly park it under his parent's windows. However, Naples and moreover the Spagnoli neighborhood isn't a place you should drive in. And indoor parking costs 70€ a night.)
2) If you have trouble sleeping at night, bring earplugs: the Spagnoli neighborhood is VERY loud even at night. People speak loudly, scooters honk every 10 seconds and cars have to rev to the max to climb the steep paved streets.)
Now that you know that, you can stay at Marcello's place and enjoy it to the fullest : he is a perfect host, his apartment is spacious, the bedroom bright and well furnished, the place is clean and cosy and it's perfectly situated in the lower Spagnoli.
Thanks for the experience, Marcello. It was great.