As a experienced DM I think staff weapon will be two-handed weapon, it is too long for one hand. It might come with reach and 1d10+STR bludgeoning damage or 2d6+STR/1d12+STR without reach. It looks pretty heavy.
It is alien ranged beam weapon so most of the players won't be profficient with it but still can have a lucky shot. Problem is players or NPC on higher lvl will hit almost all the time, so disadvantige on attacks or flat -5 for hit for example might be good idea.
For damage I think it will be force because that is the most dangerous type of damage that is similar to beam weapons by my opinion.
Damage dice? Well if players will gain HP on lvl up as they do in other d20 rpg's they might survive multiple staff weapon shots.
If Hp will be fixed then damage dice should be around max health. That way you can die in one shot, but you also might get lucky and survive
I would set it somewhere between 2d6 - 3d6 or 3d4+3
Knockback is usually STR save and for items it is usually fixed DC number. Might be around 15 but it should be auto knock if that weapon is that strong
If you would ask me about armor class, well cover in field will be really important now (half cover +2 to AC, 3/4 cover +5 to AC, total cover - can not be shot at) then there is flanks etc. Kevlar Armor might give you +1 or more to your AC but it also might give you temporary hit points (5e do have these), so if you do got hit that armor might save your life but is ruined after
I am sure it is already done by now, so I expect them to outsmart me with their weapons this summer :D