I can relate. The key word may be reliable. If you check the reviews on major sites here in the the US like Newegg.com you will find that there is an average 10-15% failure rate on all electronics orders. That is to say that whether it's whole computers or the each individual parts there is a substantial failure rate. Further if you look closely you will see that the reviews sometimes aren't actually for the listed products and you will also see reviews on different products that use exactly the same language which leads one to believe they are fake.

We do have local privately owned shops that will build whatever you want to your specifications and you can try it out before you leave the store. I don't know where you are but I suspect you have similar places at least near major cities. And of course there is a premium cost for personalized service.

Information is your friend and you didn't indicate how good of an idea you have for what you need or what you're budget is. I've spent a hours watching YouTube videos sorting out what might be a reasonable machine. And I've poured over the huge variety of set ups that people are making work.

Still the people who make the videos are mostly gamers and have little or know experience flight siming but you can interpolate because any computer that will play AAA games will run MSFS. The sim is mostly dependent on single core CPU clock speed and there has been a recent price drop in perfectly good gaming CPUs that run at pretty much the same speed as the newest generation. The picture is a little more complicated with AMD because in the past there strong suite has been core and thread count but that's mostly changed with their newest CPUs.

Perhaps the biggest problem is that you just can't get the most recent GPU video cards and the last gen cards are holding their price as result.

You do have one big advantage in my opinion. You can buy the game as a DVD set and unless you have high gig internet service it would be worth having a DVD player installed to save the hours and frustration of trying to load it from the net. It seems that you've actually hit on a good business opportunity. It would take some start up funds but there wouldn't be any reason someone couldn't buy generic computers and resell them with MSFS pre installed.

Could be a good idea to create a store, or for a store to create a department able to deliver "ready to fly" PCs

For the time being, all store propose you to configure a PC, some hints to configure a PC etc... but you are the one who must decide, configure, then make the start, test and experiment all the bugs because you're not a pc monster...

And when you're in a the countryside in south of France, local FS specialists are not so many !

With my next setup, I called for a PC specialist to change my HD and reinstall windows, but he didn't know nothing about FS and I had to contact assistance of many suppliers to fix the bugs with connecting screens, MCP, overhead etc..