Hello, and welcome to PCHF.
do you need to have a working os or can you just install freespire for os.
Why not? You can install Freespire as your only OS, or you can have multiple operating systems on one computer (depends on your hard disk space and memory).
Is there enough space on a 20gb hd?
You can check whether your system meets the Freespire requirements here:
System Requirements - Freespire