quite a lot. Gramatically both languages are similar 97%. Phonetically is a lot of difference. So, you'll be able to read portuguese without much trouble, except for some words and familiar constructions but your ears gonna need some training. The problem with spanish (castillian, in fact) is that phonetically is quite separate from the romance languages, ought to arabo-andalusi influence. But if you know any other romance language, portuguese phonetics is plain.

Yes, knowing Spanish will make it easier, but it also depends on your level. If you are fluent, you should be able to speak Portuguese within a year, and understand a lot. Of course, several variables will affect just how well you speak and understand. I would recommend this site: http://tltc.la.utexas.edu/brazilpod/tafalado/ Best wishes!

Yes, several words in Spanish are very similar and often identical to the Portuguese. What is very changes the pronunciation of words.

yeah they are much similar, and i guess it'll takes you a year or so to learn. basically that's why most of my brazilian friends are capable of spanish