After playing around with a fisheye and finding out, it´s not usefull for me, I decided to buy a wide angle lens. I searched Amazon, and found the Sigma ES 10-20mm 4-5.6, DC HSM (also recommended by my friend Matze).
The Retailprice was € 719,- and i managed to get a slightly used one via Amazon Market Place for roughly €300,-
Saving more than € 400,- was a good one
I made some testshots and I´m happy with this lens. Now I need to dive deeper into wide angle photography.