2) Concerning Windows 7 PRO, was that a reliable OS? Apparently the DELL PC would come with that as standard together with "a Windows 8 64 bits" license. Would that allow me to upgrade to Windows 8.1 (and would I want to)?
Windows Vista was a terrible OS.
Windows 7 is what Vista should have been.
Whilst 7 is a decent OS, if I were you, since you're starting from a clean slate on a new system, I would just bite the bullet and use Windows 8.1. Better to start learning now then have to face the inevitable a few years down the line. The added benefit of using 8/8.1 rather than 7 is it buys you an extended period of support from Microsoft, given that 8/8.1 is the current product, and 7 is gradually being phased out (end of Mainstream support 2015, end of extended support 2020 .... compared to 8 with dates of 2018 and 2023 respectivley)