120GB may sound a lot but it will soon fill up, with the difference between a 120GB and 240GB less than a tenner, I would go for 240GB or even 480GB. I started with Win7 on a 60GB and it was soon full, then cloned it to a 120GB which again filled up, now on a 480GB with Win10 which seems good.
Remember MS and other Application are constantly having updates and patches so your usage will only ever increase.