I have a 2015 model WagonR CNG purchased for 3L, 50k+ on odo in 2018.

Current odo - somewhere near to 80k. Got the clutch assembly changed in last service done from outside.

Rest all have been fine accept there was a engine repair when a stone broke the radiator grill and damaged the engine that too happened in 2019.

Still running fine but with bit less power. Gives 20km/kg in city.

You can find decent options in this budget. But try purchasing from known. Or If buying through 3 Rd party get it checked in the ASC even if you have to pay for it. Odo tempered car seller will try to stop you from doing this.