Hi,
First question.
What was the diagnostic from? GMATPrep? If not, it doesn't mean much as to the STATUS of your prep. GIve a GMATPrep. Your actual GMAT score is bound to be within 30 points of the practice GMATPrep.
And what should you start with? Simply put, If you are NOT consistently getting Q48-Q50 in practice tests, you need more practice. Don't read books. If your scores above Q45, books wont help. Below that, refer 8
MGMAT guides. "Foundation........" is for people who have forgotten maths altogether.
If you are getting V44 consistently, then only GOD can help you score higher, maybe come in your dreams and say you are a Demi-god or something. Inspire you to score higher. Anything below V40 you need help. Figure out where exactly you lack. Is it SC or CR or RC. Handle it using
MGMAT books. Or Powerscore CR Bible if its CR.
Regarding paid test prep:
VERBAL:
Magoosh ( The best, I used e-GMAT,
Magoosh & Economist Gmat)
STARTING FRESH PREP : EconomistGMAT (Interactive and makes you learn the stuff, repeated test takers only know)
TESTS: GMATPrep, GMATPrep, GMATPrep & yes, GMATPrep.
MGMAT, is good for practice. Don't rely on its scores, they don't mean much.