Hello,
basically I don't think the idea is bad to buy a plot of land now and only build a house in a few years when more equity has been saved up.
But you should definitely pay attention to the advice from (variable loan) and (building obligation) so you don't walk into a trap.
If you work as a consultant in the banking sector, I would, in your place, have a conversation with one or two competent specialists for construction financing - after that you should see much more clearly what is possible or not.
Feel free to start looking for the perfect plot for you - this will probably take some time, especially in the Frankfurt metropolitan area.
We, for example, searched for almost 3 years (in the Stuttgart metropolitan area).
Regards,
Dirk