Final Calc and ProCalc were probably the 2 best (commercial) spreadsheets available. They show up from time to time on eBay.
Your 2000 will run spreadsheets and most classic games and OS3.1 just fine as is. The accel and RTG were to let os3.9 run at it's best. OS3.9 was mentioned to prettify and extend your Workbench experience.
You might be better served getting some books ("The Amiga Companion", "Denny Atkin"s Best Amiga Tips and Secrets") and more experience with the OS first. Aminet has an enormous selection of free files (Directory Opus, PPaint, Hippoplayer, Ordering, Play16, PlayCD, Textview, Visage, NoteIt!, Yam, NComm, all kinds of games) for Amiga.
OSes 3.5 and 3.9 are largely OS3.1 configured with standardized (and attractive) eye candy and updates.