I also took the MDM exam and agree with basically everything in this post (very good advice)- Very similar to my preparation and test taking references. I pretty much had the exact same reference list with the addition of a Materials Science book. I never thought I would need it but actually got 3 answers directly word for word from it.I did the Machine Design & Materials depth.
I used the MERM, FE reference handbook, Shigley's Mechanical Engineering Design, the machinery's handbook, and a binder full of worked practice problems, notes, and pages from a few choice sections of college textbooks for areas I felt the MERM was lacking during the exam.
I also brought with me but did not open or remove from my crate the PPI Six Minute Solutions for mechanical systems, the latest (2016?) NCEES practice exam, PPI practice exam, MERM practice problems companion and mechanics of materials by R.C. Hibbeler. I did do all the problems in the six minute solutions, the two practice exams, and most of the MERM practice problems companion in preparation for the exam.
I also bought the PPI 'bundle' and completed ALL of the problems that were offered for the subjects that were on the NCEES exam specs multilple times. I spent 200+ hours and took 2 full 8 hour simulated practice exams. I can't stress enough how important that was. It allowed me to understand my speed. Each practice exam I took I still had a lot of time left at the end to look over problems. THis allowed me to be more comfortable bringing more references in to look through at the end. It is all about knowing yourself, your speed, and your strengths.
Online courses weren't helpful except for keeping you on track with studying. The lectures were too basic and broad - not worth the $$.. I took the MNSPE online course.