While the X-Plane team works on Macs, I would probably recommend looking into a PC. You can often get a lot more storage space and graphics power for the same price or less.
Note that the minimum and recommended specs have NOT yet been announced.
However it is clear that you will need a high-end, gaming-level, desktop (preferably) machine to run X-Plane 11 at high settings. The more powerful and capable the better. You can certainly use X-Plane 10's recommended specs as a baseline and then try to upgrade specific items, such as your video card, as much as your budget will allow.
No one can predict the future, so we can only design X-Plane with the current operating systems, graphics capabilities, etc, in mind. So in general if you try to buy the best that your budget will allow right now you will be better prepared when we do have official specs.