Hi,
Welcome to
BFG. I decided to start the game a while ago and its really good fun. If I were you I would stick with Imperial vs Chaos to start off with, as the boxes of plastic cruisers are cheaper and if you wanted to magnetise/blue tack the different gun battries on you can try out different combonations to see what works for you. For a starter game, I would possible look at about 750-1000 points, by the time you've bought captains, re-rolls and
CSM for some of your Chaos cruisers.