Hey Mike and mewton, me and one friend found one serious looking webpage, where we could buy our CSS server.
We choosed the following options and it only costs arond 16-20 euros:

Game: Counter Strike Source
Slots: 20 (With or without password you can choose that)
1GB Fast Download
Source TV 1 Slot (Dunno if neccessary)
NO Homepage ADS
+ Protection Mode
No Server ADS
No Bots
Around 500fps
Servers are located in "Frankfurt am Main"
low pings
Payable with PAYSAFECARD

Do you think this is something for us?