Indica V2 Diesel - particularly when Zen Diesel is a piece of history now.
Also Indica makes a better cab than a Zen Diesel. Even otherwise also, indica diesel as on date is a better car than Zen Diesel - maintenance wise also it is lesser than Zen Diesel.
Pay special attention on rego process as you want to use it as a cab.
Indica Diesel is definitely better than Zen Diesel. It is very spacious, comfortable, sturdy/strong & is fuel efficient (17/18 in city & 21/22 on highway) - just right for long drives/run. In fact, it's popularly used as taxi, therefore. Maintenance is also low (else it would not have been used as taxi).
You can also consider Indica Turbo - it is virtually as good as petrol car - but will cost you little more.
Indica Diesel is better than Zen Diesel, for the reasons explained before. New cars are definitely preferred over the old ones - if your budget permits.
In case of a old Indica - go for a one that is preferably not more than 3 years old and not driven more than 35/40K. It may cost you in the range of Rs.2.50 to Rs.2.75 lac - depending upon the version of the car, its condition & the city you are located in.