A 500gb ssd will hold a good number of games. That is all I happen to need.
A ssd will come in two formats, 2.5" and m.2
The m.2 devices are about the size and shape of a stick of gum.
They will plug into a motherboard slot and do not need stat data or power connectors.
Then, m.2 comes in sata or pcie versions.
pcie versions are more expensive but have higher sequential data transfer rates.
Not as big a thing as you might hope since most activity will be small random I/o.
MX500 is one of the better ones, but I like Samsung evo for performance and reliability.
If your new motherboard supports m.2 pcie, Try to buy a Samsung 970 evo m.2
https://www.newegg.com/samsung-970-evo-500gb/p/N82E16820147690?reviews=all
Defer on a hard drive and save budget until you actually need the space; it is easy to add a hard drive later.
Just my opinion.