I'd say, let them have the option of choosing their houses, but if they really have no clue, then we can always choose for them.
For the "Order" and the "DE" thing, they should be able to choose which one they want to go into. I have to say though, no spies. We do not want anyone to get an unfair advantage over the other.
There will still be Dumbledore and Voldemort, correct?