I too would say; 6Volts!! (or selectable, 5 or 6, by a jumper, though with switching power supplys this is easier said than done.. )
How are you going to make it?
A friend of mine lives in China, and he is able to make PCB's for the lowest price possible..
here is the link to his advertisement.. He is reliable and one of the most kind people I know.. (he used to life in Holland, but he emigrated to China)