I've been told I can not strip this stain off and need to sand it off. The spindles can stay as they're in good shape still. It's about 800 sq feet plus stairs. What would be the best and most time effective way to tackle this? What would be the best sander and paper grit?
Solid-color stain is very similar to paint in appearance. I'm not sure why it wouldn't be strippable, though. That said, it would take a lot of stripper for that deck. I'd probably rent a floor sander (plate type, not drum type). Use a medium grit so you don't have such a hard time removing sander marks with fine grit later.
That and a random-orbit sander for the shoulders of the boards should do the job. Eventually.
You might try stripper for the railing, though, or simply replace the balusters.