First make sure your donor nand will work, if you then flash it with the donor Nand, you will have to flash your dvd drive with the SAME dvd key the donor xbox has. Both xboxes will now have the same nand, same drive key...
You are correct about the CPU and key being paired. You will still need your old cpu key to place into your donor nand, but if you can get your old CPU key you can probably just retrieve your old drive key to start with.
**EDIT** Of course unless you have a rework station and another CPU that you know the key of, but of course that is unlikely and unpractical.