Actually the problem is everyone compare the prices and availability of parts of any car with that of mehran. Yeah prices in this way are higher and parts aren't so easily available, same is the case with chevy joy. Rather these issues are more prominent for chevy as compare to santro. There're selected shops for spare parts of these cars in almost every city.
For people like me, this isn't a big problem, you'll see hundreds of such cars in your city and every one is getting the parts. Initially it took me some time to find expert chevy mechanics & parts sellers in my city but now I know almost every chevy mechanic and parts providers of my city. If you want luxury, comfortable/smooth drive and more features then I'll surely suggest you to go for santro or chevy and obviously maintenance will be slightly costly than mehran. If you wanna prefer cheap maintenance over comfortable drive & features then go for mehran.
Mileage depends how well maintenance car is, so it vary from car to car. I'm getting 14km within city and 15-16km at highways, one of my friend is getting 16-18. I'm also thinking to do fuel treatment for my car which I hope will improve the mileage.
If you're still confused, I'll advise to see thrice of these models and buy one with best condition and latest model.