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Have a great day and remember to play a math card game!
The letters roughly correspond to grade level - level A is kindergarten, level B is 1st grade, etc. I have heard level C may take more than 1 year to complete, but I guess that depends on the child.
We have nearly finished level A, and I would definitely say it's complete enough on it's own, especially if you play the games ~3 times a week.
The necessary items would be in the starter kits. You can take a look at what's in the kit and just order what you need if you already have some of the items. The abacus is an absolute necessity, for sure!
The nice to have stuff are the recommended items, which are included in the deluxe kits.
My child tested higher, but we went ahead and did level A anyway, and I'm very glad we did! I can tell my daughter has an excellent understanding of math and she really enjoys RS. She actually gets upset if we do schoolwork and don't include math. 
It has laid a great foundation for future work, in only 5 months since we started using it.
