What are you building - a computer for ants?
You will usually run your OS from the SSD. With Win 10, that would mean that also all programs and the default user directory would like very much to reside on the same drive, that is your SSD (yes, there are workarounds, which will make trouble on other ends, so I try to avoid them). I have a 500 GB SSD running at the moment, which is sufficient - the 240 wouldn't be, although I am storing all the data heavy stuff (music, images, other documents) on a secondary HDD.
I'm currently tempted to replace that with a 1GB M.2 - at 1 GB, the price difference between the 2.5" and the M.2 in the Samsung 860 EVO series is negligible, either is ~180 €.