I am not with Laminar Research; just a flight simmer.
You question raises a few comments worth consideration.
If you haven't purchased or ordered your new computer go for a windows based system and not a Mac system, Although X-Plane is developed by Laminar Research (LR) using Macs it has been seen in this forum whereby LR staff recommend using the official version for general use on a windows system, A windows based system is far easier to upgrade individual components than a Mac system. For the same high end build in a Mac system it will cost you an arm and a leg compared to a windows system. You will need deep pockets
For a better performance when you order a new PC go for a NVidia graphics (GPU) card and an Intel processor (CPU) over and above an AMD GPU and an AMD CPU. Don't go for the new 9000 series CPU. It is an overkill as X-Plane will not utilise more than 4 cores. Have a look at the following link from Michael Brown found at https://www.youtube.com/watch?v=OWVngv3iAvY
Support the official and original developers of x-plane, being Laminar Research, and purchase X-Plane from them. The steam version is a 3rd party hybrid that is taking a commercial advantage of the development work by Laminar. It is not the same product. There is no connection business wise between Steam and Laminar Research
Whether you purchase the LR version of XP now or later (when you have the new PC) does not matter. Provided you install X-Plane totally in a separate directory or on your desktop all you need to do is copy from one PC to the other using a high capacity storage device. You don't have to reinstall on your new PC and can transfer all of your saved configurations from one PC to the other.
In purchasing the official version of X-Plane you can do it directly from Laminar Research here at https://www.x-plane.com/desktop/buy-it/ but you will not be able to purchase any add-ons. Laminar Research does not market additional aircraft over an above what is supplied in the X-Plane package.
To purchase additional add-ons support the dedicated X-Plane only website called X-Plane.org which can be found at https://store.x-plane.org/ This site has no connection with Laminar Research. I would suggest deferring any "additional items" purchase until you have connect X-Plane to your new PC so that any "bugs" can be sorted before creating any possible additional bugs. This site also sells and supports the official LR version of X-Plane.
I hpoe this helps