1. Quale versione di Windows mi consigliate in base alla vostra esperienza? 7, 8.1 o 10?
I use Windows 8.1 myself and would not use Windows 10 because of all the bad press it receives, at this very moment. I'm sure that others will tune in here to tell you that they have good experiences with Windows 10. Which is fine, of course.
2. Quanta memoria devo dedicare alla partizione Windows che utilizzerò principalmente per lavorare con Trados 2015?
I'd say that you should give your Windows partition about 100 GB of disk space.
3. Dovrò acquistare anche il pacchetto Office per utilizzare Trados? Se sì, dovrò prendere quello per PC o per MAC?
You will only need to buy MS Office if you want to pre- or post-edit your source files. Depending on how much Windows or Mac feeling you prefer, you can choose the Windows or Mac version of MS Office. Personally, I use the Mac version, since I also have a CAT tool that runs on OS X and I only use Windows to quickly verify my SDL and memoQ packages for delivery.