Hey Homefries,
Nice find. Keep at it, and don't let the detractors get you down.
Here are some observations:
1) the building itself looks okay as a framework (ladders, floors, support posts)
2) there are areas that don't work due to scale: pylons, the the fence on the roof, the bar, and the chain on the pylons on the floor.
So, how do you approach this.
Look at some of the current plastic buildings, drawings, artwork of
40k for inspiration. I'd suggest maybe adding layers of plasticard along the outside of the building to act as walls. Then you can add details/aquilas/slogans/windows, doors, etc. That will give it a more 'modern' feel.
I agree about replacing the roof. You want something metallic looking, or modern.
You can add additional internal detail, just make sure the scale is appropriate for the models. Things like sandbags along the walkways, etc.
Finally, it has to be a different color, as you mentioned. The pastels hurt my eyes
On the flip side, this would be a great starting building for Mordheim. It definately fits that aesthetic better.
Good luck, keep us posted with updates. Don't be afraid to mock something up and put it up for comment!