A solid understanding of probability and statistics is critical to following the compling core sequence (especially Ling 570 and 572), and to maximally benefiting from those courses.
For students in the final category, we strongly encourage you to take Math/Stat 394 or one of the online courses listed above in order to get full benefit from the 570 series.
A good online data structures course can be found here. It was recommended by a CLMS student who felt it offered good preparation for the 570-573 series.