I've Chevrolet Exclusive since last 2-3 years and believe me I didn't face problem about parts availability even a single day. I do order the parts to Chevy HeadOffice at karachi and get the part delivered at my location on the very next day. Besides that, we have chevy parts sellers in every city, the point is you just need to know about them. Chevy ki ride apko parts or resale ki fiker bhula dy gi. by the time you'll ride this beauty (in well maintained condition only), you'll love this.
Santro don't have that much problem for parts and resale as compare to Chevrolet Joy/Exclusive. If you ride is in good condition (no matter its santro or chevy), I'm sure you'll easily be able to sell it.
Please don't go for Mehran. Please buy anything that is really a CAR or stay without car but don't buy Mehran. Cultus is another option