Welcome...

First off, good job choosing a realistic short length. 20 seconds is feasible, whereas most people begin by trying to do a 5-10 min "short".

1. Maya is good for modelling, and mudbox is good for sculpting and texturing as an auxiliary package, but it's not always necessary.

2. Maya is good for animation. From what I've heard, motionbuilder is good for mocap, but you can also do that in maya I think.

3. Yes, realistic environments are part of Maya's capabilities, it just may take extra effort.

4. Mental ray comes with Maya, other than that, there may be helpful plugins, but I don't think there are real "must-haves".

5. There are lots and lots of threads on this kind of thing already, do a search here.

Basically Maya is great because it can do it all. Other software may help out in certain areas, but that also means you have to learn them too.