Bathroom Exhaust Fans

I wasn't sure what category to post this in, but I'm guessing you construction gurus are smart on this subject.

I have a 2.5 bathroom house. The half bath is in in the center of the ground floor with no exterior walls and has an exhaust fan. I'm planning a trip to the attic later to see if and how it runs to the outside.

The full baths are upstairs. One is 11'x7' and the other is 11'x11'. They both have one exterior wall (one is in the back of the house, and the other is in the front) and they both have a window. The bathrooms are separated by the stairway foyer and about 13 feet apart.

I'm guessing the full baths didn't require fans because they have windows. So here are my questions:

1) Is it a good idea to install fans? I am currently remodeling the larger bathroom and don't see any moisture issues so far (original bathroom; 30+ years old).

2) If I do install fans, what is the best way to run them outside? Can they both use the same vent? If the half bath vents to the outside, can the other two vents be tied into that?

3) Any recommendations on a quiet, long-lasting fan and vendors (stores or online)?

Thanks!
Chris