Darwazon ki insulation was done by opening the door from the inside. Jab cover remove hota hai to ull see a plastic layer or something. I did this with my cuore so have no idea what would be inside a mehran's door. Khair, I brought home a foam sheet and cut it accordingly to the shape of the door.
I then made another layer of plastic sheet and sandwiched the foam layer in between the plastic layers. Then I glued the whole thing inside the door. Remember, the holes.for the windows and lock etc need to be proper Warna it will give u problems while opening and closing the windows.
This not only made the car cooler, it also worked as excellent noise insulation from the outside 
Also, go under the dashboard and steering and check if all the ducts there are closed. If not then get those stopper type things nd close them. Wahan se garam hawa atee hai.
Thirdly, when ur car has been running for a while with ac, go down to the same under the steering area and feel it with ur hand. If it seems hot, like hotter than usual, look for the insulating foam type layer between the gray plastic of the cabin and the wall of the engine compartment. That insulation might be torn up or inadequate. Get another layer of the same material installed to further limit the heat.
Roof ka I have no idea as my car's roof is cool no matter how long I Park it in the sun. But I have heard some people install a similar plastic/foam layer there too. U can check and see if u really need it or not.
Then take off the layer from inside the cabin. It would be glued to the roof. And install the layer as per ur requirements.
These are DIYs so, I don't think u would require any mechanic for this 
The only costs would be the ducts costing 15-30 rs each and the foam layer which u can get a lot of for a few hundred rupees. U'll also need to stick the stuff so will need a solution glue or whatever that is called. The shopkeepers know it. Tell them chat laganay wali solution glue chahiyeh.
That glue will work inside the doors too but u can stick it with depoxy steel or something like that too.