Can I buy the new xbox myself when I'm 17 (if it can be pre-ordered), or is there still a limit to the price that can be bought at a time? It will cost around 500 or 600 euro
According to §110 BGB (pocket money paragraph) you can buy things with your own means. What is not allowed are purchase contracts with partial payments, etc.
In addition: Your parents can also cancel the purchase if they do not agree with this decision - pending ineffective.