Q:

Idiom for preparing very very thoroughly. ( take a lot of stuff for doing smth)

In Russian we have: Why did you take a lot of clothes and equipment? Are going to go to war? But what about English idiom?

A:

The classic idiom for taking a lot of stuff is "everything but the kitchen sink"1. There is an implication that a lot of the stuff will not be needed, i.e. the idiom is a bit derogatory. 
If you want to say that someone is using a lot of stuff but without the implication of packing/travel, there's also "everything from soup to nuts".
1 When we go camping for two weeks each August for a medieval recreation event, this becomes "everything and the kitchen sink". :)

A:

Idioms for bringing lots of stuff:
Particularly with subtle verbal cues and body language, these phrases could all imply:
My, you've gone overboard!

The whole kit and caboodle
The whole shooting match
The whole shebang
Lock Stock and Barrel
The whole ball of wax
The whole enchilada
The whole nine yards
The works
With all the fixin's

Meaning: everything or completely. 

Idioms for preparing thoroughly:

Cover all the bases

A baseball metaphor meaning prepare for every situation.

Dot the i's and cross the t's

Meaning: Pay attention to the smallest details in doing a job.
In the classic Around the World in 80 Days, Phileas Fogg was known to say:

The motto of the wise is: be prepared for surprises

He always seemed to have just the right tool packed to solve any unexpected problem.

Prepare for the worst

Truncates Hope for the best prepare for the worst.

