Hi Bright,
I am not with Laminar Research; just a flight simmer. I am not a Mac user.
Have a look at the system requirements to run X-Plane. The details can be found at http://www.x-plane.com/kb/x-plane-11-system-requirements/ Those intended future flight simmers who want to purchase a new computer, and starting from scratch as you are, usually build something that equals or is more upmarket than the recommended system.
If you are really desperate to purchase a Mac system do so otherwise consider building a purpose built PC. For what you will pay for a very upmarket PC will be a lot less than what you could pay for a Mac system. A purpose built PC will allow you to add or upgrade major components in time whereas with a Mac system you are "boxed in" on being able to upgrade.
X-Plane is resource hungry in the CPU and GPU which subsequently generates a lot of heat necessitating internal fans and or water cooling. Does a Mac system have any of this. Can you add it as an afterthought? What I have seen Macs are virtually a sealed unit. If you intend to overclock the CPU, as many flight simmers do, additional heat will be generated. Overheating will cause the system to crash without warning.
As a side issue, Laminar Research does all of its development work on a Mac system but one of its prominent staff members has recommended to another future flight simmer in this forum, purchasing a purpose built PC instead
Good luck with your purchase having considered the system requirements in the first instance.
Glenn