I use Maxi-Lock for my dipes, and WN for the colors that match the Maxi-Lock. If I can't find WN to match the ML, then it is all 4 ML. I use the WN because it give a nice even think band of color - but then my dipes are all about the color serging.
If you are just making diapers for yourself, you just need 4 cones of ML - and you only need the colors if you want the thread to match your fabric. If you do want to buy colors, then you only need the colors in the 2 loopers (Like someone already said).
WN is expensive, but it does give a nice look - it is totally your preference. However, I don't have the experience to answer your WN on knits question, since the diapers I sew aren't knits. Again, with the WN, you really only need it for your upper looper (but this may be diff. for knits)
Definitely hit the Joann's sale. 50% off thread is a BIG deal and it doesn't go on sale like that all that often.
Have fun serging!!