SC2Mapster

You won't cut it with only events, there is lots of  other stuff in triggers too ;)
For what you want, as you said credits are units, you need something like that:

Events
   Any Unit Enters Region
Conditions
   (Number of units in (Units in [Region]) == 1   // use unit group - number of units in unit group, then unit group, units in region, also it may be 2 instead of one dunno if condition counts triggering unit in that case as well
Actions
   Unit - create whatever you want etc.

For periodic spawns, you can either do it like this:
create global variable of type Timer (right click beetwen triggers and choose new element -> variable) lets call it TimeVariable

Put this in trigger where respawns start:
Timer - Create New Timer
Set variable - TimerVariable = (Last Created Timer)
Timer - Start TimerVariable, seconds, one shot, game time

And another trigger:

Event
     Timer expires     // if you can use TImerVariable here instead of timer (not sure bout it), do so and disregard condition
Condition
     Elapsed/triggering timer == TimerVariable  // don't know if that's required, look around for function to compare itriggering timer
Actions
     Unit - create units blah blah
     Unit - order them to move or whatever
     Timer - Start TimerVariable, seconds, one shot, etc

That can be accomlished other ways, like:
  -Change event to periodioc event, every X seconds and disregard all timer stuff. In other triggers, use Trigger - turn [spawningtriggerimtalkingabouthere] ON to start spawning, and OFF to stop it
  -Use Wait X seconds, then Run(this trigger) again.