I know you need diagnosis units in sickbays, but what is the benefit of dispensers, scanners and beds? Would some diseases be incurable without them, or do they make the process more efficient? Do you ideally need at least one of each?

Are workstations, microscopes and replicators simply successive upgrades of the workbench?

If I recall, yes those are successive upgrades, for the Research.

For the sickbay, I can't find the original Wiki for Startopia that went into details on that subject, but the Diagnostics Unit is exactly that, a Diagnostic Unit, it CAN heal you for the very basic stuff, but as it gets worse, like say in the one mission where you are a under Contract from the Greys, you do need that dispenser to heal the illness that comes in at the end.

So yes, outside of the Campaign, their are diseases incurable without the right equipment.

Only with the X-Ray Scanner can you see the big mean infection, and with the bed, cure it. It's all rough, I'm only jumping back into Startopia watching a Space Haven let's play

Thanks. I've noticed since then that patients sometimes leave the sickbay after a treatment still with low health, and go right back inside; So in at least some cases it might be as simple as getting multiple hits of healing from using multiple devices in a row.