The question I believe you're driving at is, "Do souls need to eat, sleep, and breathe?"
Really, it's up to you. There's no official guidance on the matter. Canonically speaking, the afterlife is just another plane on the Great Wheel; the destination for the recently departed, yes, but also accessible to mortals with the right magic. The god who presides over the plane gets to decide what requirements the denizens (and visitors) must fulfill to sustain themselves. Broadly speaking, I would assume they enable souls to partake in food, sleep, and breathing if the soul chooses to do so, but do not require it.
For a campaign like you describe, I would:
1) Eschew biological imperatives. No need for food or sleep. However, I would still include some form of short and long rest state of mind to manage resources. Maybe something like all PCs get a feature similar to elven Trance. The frequency and effect of this state of mind would be beholden to the same mechanics as normal rests.
2) Treat gravity the same, including falling damage.
3) Make drowning/suffocating impossible. Certain spells or effects whose description suggests respiration is a required part of being affected do nothing, unless the soul decides to breathe it in.
4) Make PC death permanent and irrevocable, as the soul itself is destroyed. That might be a little more hardcore than you or your players like, so I'll restate this is what I would do. Treat this point especially as a statement of preference, not a suggestion.