I do not know much about the online resources part (coz I haven't reached that stage of preparation yet). However, I do own a copy of the complete set of
Manhattan GMAT Guides and I own a copy of the Official Guide, 12th Edition, as well. The former can be substituted with other books specializing on GMAT but latter is an absolute must have.
That is my opinion.
Also, with the
Manhattan GMAT book (any book) you will get access to 6 mock GMAT tests in CAT format. I haven't taken the tests yet, but I've heard good reviews about it

Final Note:
Materials used for preparation are only as good as the person using them. So make a plan and stick to it. Owning the books and glancing through them will not suffice << I learned that the hard way!
All the best, and we're all here to help so do post your queries!