OK, SSD works just like RAM…except that the SSD does not lose information when it loses power.
If you don’t care much about reliability, then 128GB should be the smallest SSD you buy. That will handle Windows just fine. You can always use a (existing) spinning platter hard disk drive to store bulk files, like games.
Your SSD will be more reliable if you can bump up to 256GB or 512GB or even larger. This is because the life of a SSD is limited to number of write cycles. Smaller SSDs will use the limited write cycles faster. So all else being equal, a 512GB SSD will likely last longer than a 256GB SSD…even if you store the same files on both.
If you can afford a 1TB SSD, there is no reason not to install everything (windows, plus games, plus office applications, whatever…) all onto the SSD
Don’t ever throw away a mechanical hard drive if it still works “OK”. You can always use the mechanical hard drive to back up the SSD.