Go for the petrol engine as your monthly running is comparatively low.
Petrol amaze is an engine that really needs to be revved up. It will come alive once past 3 k rpm all the way till red line, under that it won't be so lively and hence the fuel efficiency. Average kmpl will be somewhere around 11-14 kmpl on city drives and 2-3 kmpl more on highway drives.
If you're going for the petrol engine and much more better interior and gizmos , then i would say blindly go for the Zest. It will definitely satisfy your needs.
Though there are some niggles on the Zest that are yet to be patched up, other than that everything is fine.
The under body coating will not be so effective IMO, though you can prevent it from rusting, i don't think that it will be damping the sound.I've also done the same under body and
haven't felt any remarkable change.
Damping all the door panels will definitely make a change