Seems to be very basic and software related, so I would suggest you to first jump into SMF itself and head over to the official forums at
https://simplemachines.org and get started so you will have more knowledge and experience with it, as a lot more of support with your issues and doubts.
This is since here we would provide only products related support assuming you already know how to handle yourself using SMF.
To briefly answer your initial questions:
1- Free
2- Surely, you can install it just as any other forum software.
3- I believe there is yes, check the official site for language packs.
4- Surely since there should be a Portuguese translation
5- I’m not sure what you mean, but maybe if you be more specific over there someone could help you find a mod to do this
6- I didn’t get this, our current theme is not for sale if that’s what you are asking.
7- There might be a mod that could make this possible you should search in the mod site on smf site.
Regards