Check my understanding of the progressive components for a redstone calculator

[mcShadesz]

Thanks for the input everyone. I may just pre build a double dabbler that they can use and just decode from there (BCD to 7 seg), or just go straight to the 7 seg myself.

Here is another question I thought up yesterday...
How committed do you think people would be clear through a calculator? Note: these would be people wanting to be introduced to redstone-digital-logic, like you would find on school server. Would it be a better idea to stop at about the subtractor so the "learning project" doesn't get too intense/overwhelming for them?

All input and opinions are welcome.