At your kind of running, moving away from petrol does make financial sense, but we would first look at how that 2,500 to 3,000 km a month is split between city and highway use.
If a large chunk of it is on a highway, a well-maintained used BS6 diesel can make sense because the fuel savings will be meaningful, and the usage pattern suits modern diesel engines far better. If, however, this mileage is mostly city traffic, we would avoid a BS6 diesel because DPF-related issues can become a headache with repeated stop-and-go use.
Installing an authorised aftermarket CNG kit on your Maruti Baleno is absolutely legal, provided it is a government-approved kit, fitted by an authorised installer and the RC and insurance are updated accordingly. Financially, this is the lower-risk route if your Baleno is otherwise mechanically healthy, because you will recover the conversion cost relatively quickly with your kind of running. The trade-off is some loss of boot space and a drop in performance. 
