Good buy. You'll have plenty of fun. For the floppy, there were problems with early AInc (or was it Escom) floppies. Check Aminet as there was a fix document there.
Regarding the OS. You'll be able to run 3.1 on there. But, not 3.5 or 3.9. Check out eBay, there are cheap A1200 3.1 ROMS there all the time.