"More than you'd ever want" is a common expression meaning, well, "this is more than you need or want."
In this context, the author is warning the person that there will be more than eoungh information.
"More than you would ever not want to know" is a humorous extension of this to say "I am going to give you so much more information than you asked for that you wish you never received the information in the first place." The context is that the recipient has the tidbit of answer accompanied by a large body of possibly undesirable information.
"all you never thought..." extends this further (humorously) by implying that the amount of extra unwatned information is large beyond the imagination of the recipient.
Sayings like "I never thought I'd be able to do that, but I did" etc. are pretty common. The speaker is playing with this convention.