Civil Services (Preliminary) Examination is quite easier than what you are thinking. Have a look at the recent years’ question papers and you will realize that difficulty level is easy to moderate.
As such, you don’t require coaching for Preliminary Examination if you are ready to prepare according to syllabus with strong determination and commitment.
The following articles would help you in getting some insights about preparation.
Civil Services (Preliminary) Examination 2013 – Roadmap to success
Tackling Preliminary Examination 2013 confidently