Rhod I actually run Windows so DT is the windows version, but by coincidence I have just built a Ubuntu server as I needed a Web server for a project and Windows ISS is a right pain to configure esp when it doesn't want to play ball.
The Installation is a doddle but you may find setting things up is quite verbose using command line scripts so expect to spend half your life on google working out how to do things. Ubuntu does have a desk top front end but to install anything is a right pain
If you want to set up your Ubuntu on your Windows network you will need to install and configure "Samba"
My own thoughts would be, that if you want to learn and get into Linux, then go ahead and have fun, once you learn it, you will be happy but the learning curve is steep and long . The alternative is to bite the bullet and invest in another windows licence. More expensive for sure but might save your sanity unless your a quick learner..
With regards to the windows licence on the Laptop, if it is an OEM version then it was sold with the licence to the last owner so it is quite valid to use it. So when you get the Laptop, try and do a fresh install of windows and see what happens when you log in using your own windows credentials
Steve.