You can appear for this exam once you’re done with MBBS. On clearing it, you can expect to live a comfortable and contented life and possibly even a lucrative career later on, with a lot of perks. You will be posted to any branch of Indian Govt. like the Railways , or as General Duty Doctors and at many other posts.
Now, the catch. You might find it very difficult to study further while working, as you may not have the luxury of time to prepare for a PG course and work simultaneously ( although a three government service makes you eligible for a post grad seat through the In-service quota).
The pay varies from post to post; A post higher up would fetch you more than Rs. 80,000 ,whereas a medical officer may only earn around Rs 45,000.