Basic 3.1 Workbench installation is a few megabytes. With necessities and enhancements you're looking at 20Mb or so, and most games are less than 5Mb in size, so that 120Gb drive is in my opinion completely superfluous.
Simply, fastmem is fast
for retro gaming it doesn't matter if you have fastmem or not. Pretty much all games were made for the stock or lightly expanded machines and will play like they should with or without fastmem. If you will be using WHDLoad for retro gaming you will notice shorter loading times with fastmem though, but it's not a world of difference.