Hi Milo,
I am not with Laminar Research or any other company mentioned below; just a flight simmer.
Have a look at the following link at https://www.x-plane.com/kb/x-plane-11-system-requirements/ The serious flight simmers have a purpose built PC with specifications equal to or better than the "recommended" requirements built. With a purpose built PC you can upgrade individual components as and when required or affordable. It will cost you less than a MAC system.
Because the CPU and GPU generate a lot of heat you need good cooling for the CPU and fans installed into the case. You will also need to consider a good and reliable power supply somewhere over and above 600 watts. NVidia cards will provide a better result than anything AMD. A this stage do not go for the new NVidia 2000 series cards as they don't mix with X-plane. If you want top of the range for VR use go for the 1080 series cards.
Have a look at the systems available through a company called XForcePC at https://xforcepc.com/flight-simulation-systems This company is recommended by Laminar Research as the only supplier of PCs in the USA for systems that will run X-Plane. The owner, Michael Brown, has a YouTube website whereby he produces a number of very informative youtube stories related 99.9% of the time just on X-plane.
You will find that the serious X-plane flight simmers, particularly those producing Youtube videos, run windows based systems. I haven't seen or don't know of anyone creating videos running a Linux system. If you run the "professional" version of Windows you can run more RAM than the home "version"
The current top of the range CPU is the i7 8700K series. The "K" meaning you can overclock the CPU up to 4.7 or 4.8 GHz. To overclock the CPU you need to run a "Z" series motherboard. This also allows the CPU to be overclocked. The current recommended board is a "Z370" series. I recently upgraded to the 8700k CPU and motherboard and subsequently had to exchange the motherboard to the "Z" series. I haven't tested my new system under full X-plane conditions as yet but using other software to test the combined ability of the CPU, GPU and RAM I get frame rates between 60 and 210 fps with an average of approximately 110 fps.
Although X-plane is or has been developed on a Mac system, I have seen comments in this forum from Laminar Staff recommending a windows based system. I think this answers your Linux question.
Good luck in building your new system.
Glenn