Wow, great! From the ground up huh?
Please let me suggest that unless you are very experienced at building studios and homes, hire an architectural acoustic consultant specializing in recording studios. They really are cheap compared to the mistakes you can make. really.
Anyway, I wish you luck with this. In answer to your questions... Beginning on the ground floor with cement walls is always good. I am currently designing a mix/mastering facility here with all concrete/brick construction for outer shell... Heavy is good.
So basement is a good start.
Cheers,
John