I am remodeling my bathroom and will be putting in a concealed tank toilet. I am looking at Duravit toilet with Gebreit tank.

Few questions:
1. Does the tank have to be directly behind the toilet, or can it be few inches or a foot to the side?
2. How dificult is to install these?
3. What is the approximate cost?
4. Any contractors in Boston, MA area that have done this installation before?

Thanks