Any pump that it’s electrical driver (motor) is not Ex-proof is not accepted for LPG pumping operations. It must not be used for such. Water pumps are not with ex-proof motor as such not suitable or designed for LPG operations. There are pumps specifically designed for LPG operations and kindly go for them. Safety is first in LPG business before cost savings. But some plumbers have entered into the line of gas tank installation using water pumps, if you are investing in LPG business please safety first.
Not good. A 3/4 HP water pump can work for 2.5 tons because it has brass impeller and pressure to suck low distance. Though it's not reliable but not costly like gas pump, even easy to maintain.
You can use a water pump for a 2.5 tons tank only if you can’t afford the normal gas pump for the main time. However, I don't recommend it.