City is a nice option if you want economy, but dont buy the auto(vario) one as it has a lot of issues on CNG, get the manual one as it very good both on petrol and CNG,
Swift Is nice too, but you should also consider Corolla xli/gli 08-09 model,very economical and comfortable car. Even on petrol corolla has better mileage than Swift, can support CNG easily aswell, plus has a huge trunk there too.
A clean swift will set u back around 11-12 lack rupees approx, (the manual one)
Depends on the condition of your car body, suspension, engine etc, a clean vitz with CNG will go for around 650-720k approx, 2000 Model.
Prices are only fr khi