I am planning a build soon. As for the in wall system, I would definately use the Geberit. They have in wall tanks for 2x4 and 2x6 walls. I am using the 2x4 option to save money on framing. I would suggest an access panel behind the inwall system if possible.

As for the toilet...many of them will fit on the Geberit in wall tank. I would just find one that you like and confirm that it is compatible with the Geberit tank. Keep your install height in mind if you have a short wifey like me.

Two alternative and (somewhat) cost effective options are the Toto Aquia (comes with a 2x4 in wall tank and simple actuator) for around $600.00, or the Porcher model - around $550 for everything. Otherwise, the Geberit tank will be compatible with almost any toilet, but it is a good idea to confirm as Ray said.