It might be unnecessary for you to get the Kaplan Premier book if you're going to get the two workbooks. I've read around that there is just too much overlap (content as well as practice questions).
Like lalitharamesh, I would also recommend the
MGMAT Sentence Correction guide. The guide is not only useful for getting familiar with the SC portion of the exam but also resourceful as the guide provides online access to 6 CAT exams, which are arguably regarded as the best indicator of performance next to GMATprep.
I would suggest that you first pickup the SC guide and take a stab at one of
MGMAT's CAT exams to see how you fare on the exam. Take note of your strengths and weaknesses, look for any gaps in your foundational knowledge, and develop a study plan that supplements or attends to these things.
Ultimately, the answer to your questions rests on knowing: what level you're at now and what level you need to reach.