You have a great answer... here are a couple of more things to think about.

If you are putting your twins in Level A Edition 2 - one thing you will need for your daughter that wasn't mentioned above will be the Transition Lessons (to move into Level C.)

Another thing to consider if you go with Edition 2 for your twins, is that you will end up buying both editions of the later levels (C, D, E) because the 2 editions aren't compatible. Just something to keep in mine.

btw - When I did edition 1, level B with my 2 kids at the same time, I found it worked best to have 2 Abacuses, 2 sets of Place Value Cards, and 2 sets of Base-10 cards. When I did Level C with them, it was best to have 2 drawing sets (although I could have taught them separately for those lessons). The other manipulatives, so far, we haven't had any problem sharing.

hth!