Apart from the special case of PAOC HEMS, "ordinary" HEMS is surely done with a normal AOC. To fly at night Commercial Air Transport you don't need an IFR aircraft (so long as you remain VMC), but single pilot it must have a working autopilot.
Does anyone do non-PAOC HEMS at night?? I would have thought it was difficult to land at ad hoc sites (unlit) at night within the AOC rules.
[The above refers to UK, but then I think most everyone else on this thread was talking UK too]