Porto is a small city but really well worth the visit loads to explore and enjoy. Once you are in the central areas most places are walkable making a metro card unnecessary.
Levels of personal safety are high, however If you have a car with non Portugese plates do not park over night on the street, the likelihood of it being broken into is high.
Winter weather, may not be as cold as northern Europe, but it rains a lot. So best to visit in warmer months or bring an umbrella.