Hi Matt,
Have a look at the components mentioned in the following link found at https://xforcepc.com/english/flight-simulation/flight-simulation-computers/budget-gaming.html Obviously there is no mention of a mother board but there has to be one installed. The recommended unit by this business is generally a Gigabyte Z370 series unit.
This PC is, as the heading indicates, a low end budget unit and is slightly higher in price than your 800 euro when currency conversions are made. It will run X-Plane. It is being marketed by XForcePC. Laminar Research recommends this business as the only supplier of PCs in the USA suitable for X-Plane.
I would suggest using the component listing as a guide for your build. To make it cheaper for you, you may have a "mate or friends" that can help with the physical build. Alternatively if you live in the USA purchase from them as the PC will be fine tuned for X-Plane when built a delivered.
The owner of the Business is Michael Brown who produces a lot of YouTube videos regarding PC components and specialises in units for X-Plane. The videos produced by Michael can be found at https://www.youtube.com/user/col2mab/videos
Whatever you do stay with NVidia Cards and Intel CPUs for a better performance. Other types of systems will give you a lesser performance.
Good luck
Glenn