I'm a big fan of the E39 5 series as well. They're damn hard to find in Pakistan, the main reason being that they hail from the era before liberal car imports began in 2003-4, and most of them would have been too old to bring in under the import schemes.
Having said that, I do see a couple of really nice ones here in ISB (i'm sure there are many more in Lahore and Karachi) on the roads. Very few come up for sale, and if they do, either ex-embassy cars or low spec base cars.
Would advise you to go for a 525i - best alrounder by far - if you can find it, or a 530i if you can afford the fuel. Wonderful machines. Way better than current 5 series in my opinion.
Try Dewan Farooq used car network and look out for embassy/consulate sales.