CD 100 will resolve both your queries....I filled my bike with fuel worth of Rs 400 at the time of purchase..The odometer has done 225 Kms till now and it still going without reserve giving me roughly 58Km/litre.Also in terms of comfort and road grip CD 100>125..I wont b budge myself with any 70 cc bike but If you want an entry level bike with economy as you are a student CD 70 would suit you best,70 is recommended for short trips.For long trips it will wear you out and CD 100 is recommended for long trips.
Cg 125 is eyecandy for thieves and snatchers(you will have to get this ride insured)and its too powerful..Just power and causes the most accidents on the roads.
If you are interested in an Honda make/model always buy from a good/popular antenna 5s dealer in your vicinity.