How hard would it be to produce a circuit that ensures that a battery (6S Lipo) does not discharge below 18V. ie so once the voltage of the cell reaches 18V, the device switches off.
It's actually harder than you might expect.
One problem with circuits like this is that the battery falls below 18V under load, so some (unspecified magic) turns the circuit off. Immediately, the battery voltage rises again and the circuit is re-enabled. The current rises, the voltage falls, back off again. No load, higher voltage, on again. The pattern repeats.
There are 2 ways to handle this.
The first is that the circuit is switched off in a way that can't be automatically turned back on again. This allows the user to try again, at which point the circuit presumably switched off (either immediately, or after a short delay).
The second is to have the circuit switch off at some voltage, but require a substantially higher voltage to turn back on again. So it might turn off at 16V, but require 18V to turn on again (automatically, or manually).
The former is simpler, the latter is suitable for things that turn on automatically.
However, not wanting to discharge LiPo batteries below a certain point is a pretty common requirement. I would be looking for power management devices to do this first. My second option would be to think about designing one.