1. Yes it is a legal and legit Windows. You use your serial from the sticker on the side of your machine to install it. So it will be fully legit.
2. Yes it will require the original serial.
3. No Office always has and always will be an additional cost. Only if you bought it on special with a bundle from the manufacturer will it come included. Most of the time those are online bundles or it comes preinstalled. You would have to use the factory restore method to get Office back.
This exactly. Like I said, the links are from Digital River, a Microsoft reseller. The ISO's still require a legit serial to activate.