Sony Xperia Z — New Android and Z5 User...

Morning all,

I have a Z5 being delivered tomorrow
I've been a Windows Phone user for the past 4 years, and prior to that iPhone, but never android - this is a new adventure!!

I have a few questions below, but mostly i'd just like people to post any tips for a new user that might be specific to the Z5 - there are loads of 'new android user' guides around but they all seem generic and quite low level...

Things that i'm interested in:
1) Is marshmallow out yet? If not, when can I expect it and how will i get it
2) What is rooting, and what benefits does it really give me?
2a) Can I install stock android on the Xperia Z5 by rooting? People seem to tell me that Stock is better than the stuff added by some manufacturers...
3) I'm primarily a microsoft services user (Outlook.com Email & Calendar / OneDrive / MS Band) - am I going to be able to as fully integrate these things as they were on Windows Phone?

Thanks!