If you want to buy a car, use it, enjoy it, and keep it for at least 5 years, I would go for the Optra, and have it insured. Right now parts wont be that big of a deal and surely not as bad as a Japanese imported car. Chevrolet is trying to move into the Pakistan market, plus its a new car what parts would you need for the forseeable future. The only problem is resale, but in the long term that doesnt matter much either.
Previously I would have suggested a civic, but with the price so high for the top of the line model, and on top of that you want to put CNG in it, that rules it out as its a new model, the cng convertors dont have much experience with it, and the engine bay looks so tight not sure where you would even put the cng thingie in!
Local made corlla - altis, xli, gli, whatever their all the same in the end. I have an SE-Saloon and I never noticed the noise but then again I play music all the time. Thats how Pakistani cars are though, you could have your own insulation put in if you really wanted too not a big deal.
My opinion, get a Japanese assembled Liana. Plenty of parts interchangeable with the local Liana, insulation, japanese quality, and resale after 5 years wont be that bad. =)