Help with new army

Hi sorry if this is an annoying beginner question, I do plan to do to a GW store to ask but I don’t want them to talk me into buying things I do not need.

I am new to Warhammer (used to play as a child but not for 20 years) and have just got myself the Astra Armageddon box and have just preordered the Ork/Space marine Armageddon box.

My questions are:

1. Can one have a game with the units in the Astra Armageddon box vs the Orks in their Armageddon box? Like for example two combat patrol boxes are comparable - are these?

2. if the answer to the above is no, what extra units would I need to get for either side?

3. in general, what extra units should I get for an Astra army (or Ork or space marine army) to be able to play a decent game?

Many thanks