Welcome to TAI
. As you said you are a small time trader, I would like to know that do you need all the space of omni to load your goods, or lesser space would be sufficient ??
I am asking this as there is ample of space in the Maruti 800 if you remove/fold the rear seats. Maruti 800 is easier and cheaper to maintain as compared to omni.
As you want a used car. Most of the secondhand omni's have been used for commercial purposes and the engine condition is not better. If you buy a 800 it would certainly be a private vehicle.
If you buy a Omni Cargo (The one with no rear windows) then you may have to get yellow board RC.
There are various things to be checked when buying a used car. Do take a mechanic while testing the car.