I think you're ready. If your goal is 650+, then it shouldn't be a problem.
I've taken GMATPrep,
MGMAT, Kaplan, gmatclub challenge CATs, and etc..., I heard that
MGMAT's math is bit harder than real GMAT
It depends, but if you haven't maxed out on Quant, you can probably use the time to improve on quant. And do many GMATPrep CATs. Their question pool is pretty big, and it will be a good practice even with repeat questions.
Good luck on your test!