var %herbtypes potion,hi-potion,x-potion
  var %herbvalues 5,10,20
  var %x $findtok(%herbtypes,$2,32)
  var %y $gettok(%herbvalues,%x,32) 

using something like this you can do what you asked for, make a list of herbs and associate a healing amount with them. If you want to only have 1 person be able to do it then just do the same thing you do everywhere else either if ($nick == Susista) { do something } or if ($nick != Susista) { return }.

As for your %health [ $+ [ $nick ] ], obviously you'd need to rewrite your Do_heal alias to work with w/e setup you pick for your herb list.