I will arrive at Delhi Indira Gandhi International Airport at 2:45 AM, I plan to book my next domestic flight to Leh with GoAir which will depart at 8:30 AM. Is 5 hours enough time to clear all the customs and go to the domestic airport?
Customs at Indira Gandhi Airport, New Delhi is fairly fast, especially in very early morning in your case. I have been done International to Domestic Transfers in 1-1.5 hours. Since yours is on non-peak time, it should be even faster for you.
Unless your incoming flight is significantly delayed, you don't need to worry about this.