I am not from Laminar Research or XForcePC; just a flight simmer from downunder.
There is no system "off the shelf" that will give you the "100%" you are "demanding" X-plane has to be configured to give you best performance as there are so many variables. You have to do the configuring to meet your requirements. Even between exact same built PCs. Internet connections will have an influence as well.
Start with the video link mentioned by disc in his answer. When done have a look through the link at https://www.youtube.com/user/col2mab/videos also from Michael Brown. There is a lot of useful information for you to consider when you configure your PC.
Michael operates a business in the USA called XForcePC and specialises in building PCs predominantly sutable for X-Plane. His company is recommended by Laminar research as the preferred PC builder in the USA. For Laminar to do that Michael's company must have some credibility.
Have a look at the following link found at https://xforcepc.com/english/flight-simulation/flight-simulation-computers.html to read the specs of 3 PCs from XForcePC that will run X-Plane that will suit your finances. Michael "fine tunes" each PC sold for X-Plane but he will not sell out side of the USA.
If on the otherhand you live in Europe or the UK have a look at this site found for X-Plane suitable PCs at https://www.wired2fire.co.uk/flight-sim I understand they have some credibility in knowing what is required for X-Plane.
Good luck with your purchase.