I woun't advice you to go for this Baleno. Factory fitted CNG makes sense. But after market CNG if not properly installed would cause you a lot of trouble throughout your ownership. Spare Parts availability would also be sometimes an issue.
If you are not specific that you want a sedan its better to go for a hatchback since you have mentioned that you would require this car for the purposes connected with your job within the city
Hatchbacks are easy to drive as well as park in congested parking lots.
Since you want a car with less maintenance cost stick with either Maruti or Hyundai.
Maruti Alto LXi(plenty are available in second hand market), Maruti Zen(if you could find a diesel one with less on ODO it would be good), Hyundai Santro Xing(good car to drive in city).
Regarding evaluation , seek the help of some friend who is familiar with cars.
Do post in this thread regarding the car and the details.
'ians will help you