Bluebell’s point is good Liz, you may need only half that especially with 3 or 4cm module trays, for example, and its worth googling module tray to see. I like the Gardman 60 cell tray but its not that easy to find.
Yes I recommend you go for staging, drainage is vital and staging is a nice balance of drainage and retaining moisture. Avoid stones as they can harbour slugs. Have fun.