To be vey frank, the IDSI is more than sufficient if you're not into mods in the future.
It will save you heaps of money 'cos the FC is great.
I've driven both IDSI and VTEC before driving the NHC.
The IDSI is very frugal on fuel

1- highway drive, get the vtec
2- since you are driving it on the highway, more than city drive, its much the same with the idsi.
3- i dont know. sorry=)
edit: oh yea, btw, is it normal for the car to vibrate? when it starts from 0kmph? mine is a city 03 btw.

Answring your questions in my POV..

1. Go for VTEC - you will not regret it.. IDSI is the most economical, but trust me, when you feel VTEC, you don't want to go for IDSI anymore.. of course it will kick in a few extra bucks for petrol.. but seriously, it's still one of the best mileage cars ever built.

2. not that much, i've seen some VTEC consumption lesser than those with IDSI - go for VTEC man!

3. Try teamcity (do a google search), mudah.com.my or go visit your 2nd hand car dealers and pick one for yourselves! Oh, i reckon i saw a forumer here @ garage section, automotive, trying to sell off his city for quite some time (modded, very leng!). Go ahead for that man, not sure if its idsi or vtec though..

P/s : try find one with extra accessories (eg. rims, ICE, tint)

all the best