They've just gone to a new version if you're buying from Singaporemath.com
I don't think you'd get much from just reading the student book and doing the activity book. I think if that were your approach, you'd be better off just getting science books from the library on a topic.
We used MPH 3/4 and are in 5/6 now. I love the material, but I think you'd lose a lot of the challenge by not using all components. I think the text/activity alone is weak. Do activities/discussion from the TG, read additional books, do the HW, HOTS, and Tests, and I think it's a seriously solid foundation.
You might check out Building Foundations of Scientific Understanding (BFSU). It's also a good bit of work, but it's much cheaper (especially if you get the ebook).