Hi "PRRguy", please listen to the advice Alex posted above. I know it's a big invest -ment and on top of that your are chomping at the bit to get your layout going.
But, and this is a big but as Alex stated above the cost of ripping everything out to fix it later is not only a waste of money, but heart breaking to tear down all your hard work.
What I did was build a small layout just big enough to run my scale stuff , say seven by ten feet. Construct it so it can come apart and go into storage when you are ready to spend the bucks to do the basement right. Get the work done by professionals and then build your layout however you like.
Oh yea, do a radon test before you start work on the basement. People fail to think about this and it can be critical to your families health.
Also, as stated above get three or four estimates from reputable builders and check the Better Business Association and Angies list.