Ok, I realiseI can use 5v external supply for the Arduino as well. I was reading about the recommended 6v-12v.
Ok then I have no problem:)
As for the adapter I will use a normal one:
e.g. in UK
I believe this to be sufficient, because all current numbers are for peak levels. So I am 250 mA short if all boards peak at the same time. I will give it a try.