fahrrad.de is the site secure?

Better go to a bicycle shop in your vicinity, where you can also test-drive and get the bike adjusted to your body!

In general, when one hears of offers that sound  too good ot be true, then it is often a scam of some sort.

But I have heard of Fahrrad.de. They have one of the best know sites for bicycles in Germany and as an advertising strategy often have specials at incredible prices for a few specific models. You have to decide for yourself if it is really the bike you want but I wouldn't fear that it is just a fake site or scam. But of course neither I nor this site can give any legal guarantees. This is just an opinion to the best of my knowledge.

It's a reputable site with physical locations across Germany.  I wouldn't be concerned about buying from them, they have 30-day returns if you're not happy with the bike.  I wouldn't say it's necessary to go in for a fitting and test ride as Beppi suggests, not on a entry-level bargain basement bike.  A sizing chart should suffice and basic adjustments can be made with some tools and watching some good YT vids.