Thanks for all your input, that really helped me narrowing my decision. :)
@Golden:
Hi, the days of paying over $200 for a basic setup full of useless and power hungry components are long gone, this can handle your situation at half your expected cost:
https://forum.pfsense.org/index.php?topic=75415.0
Fanless, noiseless, dual lan, wifi, ssd, ram, 17w power consumption, < $200.
Just search for c1037u or 1037u on Aliexpress.
Wow, that's really a nice box.. I think I will get that one.
I will use and old hdd for that and additionally get:
Intel Wireless WiFi Link 4965AGN Mini PCIE Wireless WiFi WLAN Card 300Mbps
Kingston ValueRAM SO-DIMM 8GB, DDR3L-1600
Will be in total around 210 Euro without shipping. Looks great to me considering it's fanless, noiseless and not heavy in energy consumption.