I haven't travelled to Kathmandu from the US via Delhi, but I have made frequent trips between Kathmandu and Delhi. Customs in Delhi is hassle free. They don't bother checking your bags. There are just too many passengers travelling through Delhi airport these days. Spending a night in the airport could be a problem -- it is just too small. You could spend a night in the city. If you are going outside use the prepaid taxi. Their stall is on your right just as you exit the airport building. Don't use the ones inside the airport building.