The National Emergency authority goes well beyond the narrow issue of building a wall. This will also allow Trump to overrule the nonsense in the appropriations bill that will pass today. The restrictions and limitations of that action will only apply to the monies appropriated by that bill which is only for the next 7 months. Trump can easy get around those restrictions by reprogramming other funds. The Democrats and the media will squeal like stuck pigs, but Trump but will be able to, you know, trump them.
And Trust in Trump
...and if he has the military build it we might see it being done at a much discounted price vs. commercial contractors. I wonder how far a few billion would go if done efficiently.