Partially answering my own initial question:
You must take the units from the base game. It does not have any land units.
I am not certain what to do with the single Attack Expansion I have. I could
- Attempt to sell it
- Toss all its units into one of my 2 copies
- Split its units between my 2 copies
I am not sure what is the best use of what I have. Any ideas?
I would split them between your 2 copies