On my laptop is Word, PowerPoint, … Already installed, it is enough if I buy a product key and if so, where can I buy this one by one?
Yes it is enough you can simply buy a product key. Eg Media Market Amazon etc.
I would recommend Amazon because it is the cheapest
There are two versions of Office, see https://products.office.com/...buy/office
Microsoft Office 2019 for a one-time purchase, without updates (Word, Excel, PowerPoint)
Office 365 subscription (monthly / yearly payable) with additional 1TB OneDrive memory, Outlook, etc.
Both first and second you get reputable both at Microsoft and other reputable dealers (Amazon, Ebay). But please always pay attention to the reviews, there are many crooks around, the "alleged" Keys sell. Another point is OEM licenses (is usually tuned): These are completely legal in Germany.
If you do not want to spend money: good alternatives to MS Office are:
Softmaker Office
LibreOffice
