well, as any faction my priority goes to iron producing provinces, that I make my army making provinces : there are 4 in the spanish peninsula, + Tuscany, Walachia (or close to it), etc.
Obviously the ones with mines bring a steady income so are a must, and about Serbia, it also has the advantage of bringing a title with +1 in command. Trebizond brings +2, for that alone it's a must. Then of course Trade monsters as Venice and Constatnt. are fairly enticing if you're in the trade/build strategy (which I always do).
Last point of interest : startegic position, as Morocco to keep entrance to spanish peninsula, and of course flanders