I think if you're going to do warband design you'll want double of each sets. This will give you 4x of most staple Order cards, 4x of the core monsters and 2x of everything else. You'll find a few cards or monsters you'd like a third copy of, but it won't be as important as getting a full 4 copies of the staples.
I have a considerable d&d miniatures collection, so there are quite some alternative models I use (eg for the cormyr wizard) so all I do is buy 1 warband and just copy the cards (multiple creatures and order cards), sleeve them and I'm good to go
this is sufficent for playing in a private playgroup and I don't see how this could rise any legal issues
Rob, is there any overlap at all with the previous D&D mini game in terms of numbers like HP and attacks? I also have a big pile of D&D minis and their cards and I'm wondering if / how I can integrate them into a Dungeon Command game.