Hi, your price range will get you in the lower spec Alu frame FS models of most brands. The Canyon CF7.0 and Trek Slash 8 are both beyond your budget, unless you buy them used. The Canyon CF has also a carbon fiber frame.
It sounds like you want one bike only. Try to figure out what the travel sweet spot is in your area and for your riding style.
Since you want to keep the bike over a longer time and upgrade. I would buy the best frame that you can afford.
Since you are in Germany, check also out Rose bikes. Also check your local bike shop!