Hi there,
I would recommend sticking to Official GMAT materials. They are the best possible thing to work on and there is a sufficient supply of them.
Most people don't need third party questions or practice tests. There are certain situations in which third party questions are helpful:
1. You've been studying for so long that you've run through the official stuff three times and so need some new questions to chew on.
2. You are really gunning for a perfect Quant score. Not a 45, 46, 47.... a 51. In that case, some extra tough GMAT club quant can be helpful.
For test scheduling, I'd recommend a test somewhere in the beginning of your prep so you can get an understanding of what you're up against. Then I'd hold off for a bit to build up some skills. In my mind, you get the most out of the practice tests a little bit down the stretch after you're well acquainted with the exam. But, leave yourself at least a month to work through the official practice tests. The GMAT prep tests have a ton of excellent questions to learn from. And, let's say that you're starting sub-500 and want to achieve 700+ you can probably mine the practice for much longer than a month.
Happy Studies,
A.