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?