Checking for a building.

I want to make it so, when the worker is order to build a building, it checks if there is a already existing building there(doesn't require buildable), and if its in the same position(and not one off, scout-tower size).

So my questions...

• 
  [li]For the build order (Custom_ID) is the target point in the center or at the bottom left or etc?[/li]
  [li]If you check for units within a region smaller than the actual unit, will it still come up?[/li]
  [li]What is the size of a scout tower in distance terms. ( I think its 130 x 130, not sure)[/li]

1) I believe it is from the origion point wich is at the center bottom.

2) yes

3) look on world edit ~.~ or if you cant find it make a region that is the same size