Jessica,
Not sure which articles are missing. They've been swapping out alot as Kim noted and some articles are very old now and have disappeared. Not sure why though!
You can take a look at my Guide - although it does not cover the newer features: http://www.marketingrockstarguides.com.
I'd say maybe 70% is useful as you go toward certification. The thing with getting certified is you do need experience with the system before signing up for the test. I have seen several people try to do this test after4-12 weeks of "studying" only to fail.
I would go through the course in this way:
1. Basic Videos
2. Try stuff in Marketo
3. Get stuck? Go to Intermediate Videos
4. Try stuff in Marketo
5. ready for more interesting stuff? try next videos.
At this point, I would come to the Community and do searches for particular questions you have. There is a video, article, or discussion about your question.
My sense is that you need about 6 months of training and use to pass the test. Using the system for 1 year and a little bit of studying will also work.