Hmm. I'm not really not the best person to try to answer, but it looks like no one else has been very forthcoming so I'll help as I can.
1. I don't know, but the truth is, it doesn't really matter because...
2. ...as you are both new, your focus should be on learning the core rules, and the literature inside the box is designed to do just that. Afterwards, when you want to start playing more advanced games, you'll want to pick up you
respective codices - in your native language, of course

. These will contain details of all of the units available for your armies, as well as the costs of each unit used to balance games. You may also want to pick up a copy of the
Warhammer 40,000 rulebook, as it contains details and scenarios for matched play games, which are intended to be more balanced and even between each side, or better yet, pick up a copy of
Dark Imperium, which will both expand your armies, and comes with a copy of the rule book as well.
3. After reading through your respective codices, you'll have a much better idea of how you'll want to expand your own army.