I'd recomend installing steam on a drive with more available space.
While Steam can install its own store games to other drives as well these days, this cannot be done with mods by default, since they will always look for the steamapps/sourcemods/ folder to install to.
If you cannot have at least 5GB of free space on your C drive, you won't be able to use the installer as intended by us.
A possible workarround would be extracting the server installer Enzo linked and hardlink that folder to your sourcemods folder.
If you're not familiar with the command line stuff, there are also tools like hardlink shell extension that allow you to create hardlinks through right click context menue.