You won't get a more central spot in Porto than this apartment. It's just a short walk from many of Porto's main sights, its bars and restaurants. It's clean (looks brand new) and well appointed - a comfortable double bed, cozy lounge with TV and a well-equipped kitchen. Spotless bathroom.
And Joao is a superb host. Not only did he meet me at the station and drive me to the apartment, he reversed this procedure on my departure. He also sat me down with a map and showed me a few of his favourite spots.
Note: this apartment is on the fourth floor and there is a lift, but strangely you must climb one flight to reach it. Still, if you can cope with Porto's hills, this should pose no problem.