Hey there, Nokah.
Well basically, if it's going to be just for your OS and programs/apps, I'd say a 120GB should be OK. This should be sufficient depending on the software you plan on installing on it. An SSD will greatly boost the performance of your system, so it's a good idea to get one if it fits your budget.
Hope that helps. Please let me know if you have any questions.
Boogieman_WD