As a faction, one of the things it will spawn from is the StartingUnit.
The StartingUnit is an omni-present dozer that can build ANYWHERE(given the space) in the map.

To make it so that you don't build anywhere, you don't actually build the building you build the scaffolding object, in which at completion, replaces itself with the actual building.

The scaffolding object comes with an auto-kill weapon on itself that triggers a unused DeathType(I used EXTRA_2), that spawns two objects, a refund money crate and an invisible unit that autodeletes after 500 milliseconds, long enough to capture the refund money crate.

I also made sure that the scaffolding object does not create rubble when killed through the DeathType.

Now, the buildings will provide a healing aura SPECIFICALLY only to the Scaffolding Object. I gave the scaffolding object an 'OBSTACLE' KindOf, and it's what buildings look for when preventing the auto-kill weapon.

The Area you can build on is temporarily(until I find more efficient way) indicated by a ParticleSystem, that draws a circle around the building.

Now, how do you select a omni-present dozer?
Right now, I have it on the Command Center- it has button to select the dozer.
I can easily put it on the Shortcut Buttons. Or, you can select it via the underused 'Idle Worker' button. And even put down a hotkey for it.

One major thing we need to prevent though is that PER PLAYER, there only needs to be ONE omni-present dozer. Which is highly unlikely due to the fact this object is only created at the start of the game on the faction you chose.

Creating a Command Center/ConYard does not spawn another one. Every building just requires a Command Center/ConYard. To make it even more "fair" I suggest the Command Center/ConYard be done via MCV deploying. Which needs a War Factory/vehicle production.
You cannot build a Command Center/ConYard via omni-present dozer.