Chevrolet parts (all of them) are easily available in Karachi dont know about Islamabad.
If I had to chose between these three Platz would be out immediately and my preference would have been Optra :). Test Drive both and see.
Even if you sell after 4 years Sunny would sell better than Optra or Aveo and you would be in a better position to source parts if it breaks down in some small town. Chevrolet parts wont be available in every city.
Since resale is not an issue, between Sunny and Optra you should chose whichever is in good condition. Both are pretty good cars and wont give you any trouble if maintained properly.
Optra might give you slightly better mileage than Sunny.
Look at both of them and buy whichever is in good condition and reasonable price.