Well in my personal experience my hybrid isn't really bad in hilly areas and snow.
I have 4x4 and 155hp gas engine which is quite good for my needs.
I think the prius comes with a 70 or something hp engine with about 100 or 110 nm of torque which wouldn't do so bad in hilly areas. I think that is higher than a mehran etc, so you should be alright. Over 35mph I know that my truck starts using the gas engine so you would have to look at how the prius hybrid functions.
The cost's part I was referring to was that I do not know about the parts availability of the Prius in Pakistan. Second I do not know how much the Prius costs in Pakistan.
Let's say you purchase a used prius for 15 lacs. it has about 100,000km on the odometer. Who knows when you will need to replace the battery pack in the vehicle, which is about 2000-3000 USD here in the States.
Do a cost analysis. Take another vehicle you are interested in then add up potential fuel costs, part expenditures etc for those vehicles (Best estimate), initial costs, depreciation costs (Potential Resale) and decide on a purchase.
Personally I would not get a Prius in Islamabad. The potential fuel savings are negligible, but then again you may disagree if you foray into Pindi on frequent basis'.