Sam,
There are many expats living in Pakistan in general and Karachi in particular.
karachi is a city of over 10 million people so it is bound to have crime associated with the urban sprawl.
If you are coming as an employee of a multinational (I am sure), you will be provided all the required assistance as well as security guards at your residence.
House, Car, driver, maid, gardener, helpers etc are all part of the expat perks package usually given.
You should get a house with it's own standby power generator as power failures are frequent and prolonged in summers.
Defence Housing Society / Clifton / KDA-1 are the main residential areas for expats.
Anything and everything is available in Karachi from brand names to third generation fakes.
I am sure you will have an interesting time in Pakistan.
Drop me a message if you require information on anything in particular.