I will never again build without a ssd for the "C" drive. It makes everything you do much quicker.
120gb is minimum, it will hold the os and a handful of games.
But, many things default to the "C" drive.
When a SSD nears full, it will lose performance and endurance.
240gb is the recommended minimum.
If you can go 240gb, or 500gb you may never need a hard drive.
You can defer on the hard drive unless you need to store large files such as video's.
It is easy to add a hard drive later.
Samsung EVO is a good choice for performance and reliability.
m.2 is a size format.
About like a stick of gum.
It is nice because there are no cables involved.
m.2 will come in sata or pcie flavors.
The advantage of pcie will be faster sequential processing.
That is a plus, but you generally do not notice the benefit. On virus scans perhaps.
I like Samsung for performance and warranty.
At the 500gb size you can get the
860 evo for $130 with a 550 MBps sequential rate.
The m.2 860 evo is $5 more with the same 550MBps transfer rate
the 970 m.2 evo pcie will cost more at $210 with a 3400 MBps rate.
You can hold a goodly number of games on 500gb; that is all I have.
With today's better ssd prices a 1tb drive is reasonable.
Use a HDD to store large sequential files such as videos.