Is the trigger area a circle and you want them spawned randomly inside it? That's already baked into the createUnit and createVehicle commands!
_unit = group player createUnit ["SoldierWB", position player, [],
100, "FORM"]
_veh = createVehicle ["Cobra", position player, [],
100, "FLY"]
The number in bold there is the same as "placement radius" when you create the units in the editor, i.e. the random area around the "position player" (the centre) in which the unit will spawn. So just use position trigggerName or invisibleH1 or gameLogic1 or whatever as the centre and spawn away! Since you already have the spawning script, you might have to edit it, but it's not a very difficult edit.
If you've got an esoteric trigger shape and actually want to spawn things inside it by necessity, well, that's going to be a little more complex. Unfortunately I can't help you with the math of that
Kronzky uses it in his UPS though, so if you google that you might get some pointers?
As to killing enemies who enter a certain trigger area, there are plenty of scripts that do these kinds of things for you, but the simplest would be:
Trigger, x-y = area of death. Activated by: [enemy side] Present, Repeatedly. Condition: this, on activation: {_x setdamage 1} foreach thisList
That should kill anyone of [enemy side] entering the trigger area.
Wolfrug out.