Have you taken a diagnostic exam yet? I think the best first step is to figure out where you're starting and what areas you are going to have to focus on the most.

I've been using the Manhattan guides and Knewton. I think they both cover the exact same content and strategies, but Knewton is simply more entertaining. If you have the motivation to stay focused on reading the MGMAT books, then I think you can forget about Knewton and save your money. For me personally, I like the feeling of seeing and hearing someone explain concepts to me. Knewton does that.

Don't waste the OG problems until you've put in some solid studying time and know what you're doing. If you follow the Manhattan guides, they'll tell you which OG problems you should be doing at each stage of your studying.

Once you've gotten the foundations, the Powerscore and Aristotle books are good references to dig deeper into your weaker verbal areas, if needed.