Since mehran runs on a leaf spring system, it is hard at the rear. Putting extra weight like a cng cylinder helps a little but nothing more can be done.
Adding power windows is not a problem but usually such modifications dont last in a mehran because of its rough drive. Its best to keep it simple. Rear seats are fine, replacing them with larger seats will kill the rear leg room. You may change the fronts with buckets to enhance rear seat leg room.
If you are really bothered about rough drive, get a coure or any other car which does not have leaf spring systm.