Straight from our CEO's mouth (well, keyboard):

"They are the same tests, and the seeming discrepancy you note is indeed there. The tests are priced to encourage book sales, because MGMAT thinks that if you buy one book, you’re likely to buy others. Also, if you buy the tests you can access them immediately as opposed to waiting for your book(s) to arrive. But yes, they are the same tests, and it would take someone very impatient or price-insensitive to buy the tests instead of a book! "

The Question Banks do not overlap with either the CATs or the Challenge Problems. The full Set of 8 Question Banks completely overlap with the individual question banks, since they're the same thing.

All of our products have full answer explanations.

Your firsts question said
"If I go for Complete 8 Set will it have the same questions as in $39.00 package"

I'm not sure what $39 package you're referring to - can you clarify?

Also, please note that if you buy the books themselves, you will get access to various parts of the online materials for free. (Eg, if you buy the Sentence Correction book, you get the SC Question Bank and the CATs.)