SC2Mapster

@KratsAU: Go

When you simulates searching enemy in trigger, you do some sort of:

Pick each unit in region entire map, with filters... for e.g. player = pc
Set variable AttackerUnit = picked unit
Set variable MinHP = 1
pick each unit in region (circle at point position of AttackerUnit with radius 6)
Now we will do priority sorting. For example i'd like to select target enemy with lowest percent hp.
1. Get picked unit property hp percent current
2. if picked unit property hp percent current < MinHP then
Set variable MinHP = picked unit property hp percent current
Set variable TargetUnit = picked unit.

after this sorting, we will receave TargetUnit with smaller hp percent. and we can order AttackerUnit order Attack TargetUnit.

4.Changing a units Target - Attack Target Priority outside the data editor

This is possible by triggers
You can simulate searching enemies around unit and use your priority rules to take the target you need.
Then you can order your unit to attack target.

So this is possible by using only trigger.
If someone need detailed step by step tutorial, i may help.