PECHS is very good in terms of long term investment. In fact its location is excellent. You are right, nowadays due to Airport rumor, the prices are stagnant and buying/selling is bit slow but in my opinion, this is very good time to hold some property in K or M block. Specially i'd recommend to buy in K block as it has given the possession.
You keep one year in mind, then you will see the return.
The rumor about Airport shifting will be clear in this time. Still if airport had to shift in Rawat, the work on this airport would have stopped but in actual it is very fast. Thousands of workers are there and work 24 hours to complete October 2016
And once this airport is operational, getting plots in PECHS in these prices will be no more possible. Also the rumor about airport was purposely spread to boost the prices of Bahria town. Even if you check the Bahria town Market, the prices of plots are increased by dealers only. There are very less genuine buyers in Bahria market also. If you look at plot prices there, you will see it is just a game of big dealers. But soon it will come to end and things will be clear. In PECHS, genuine buyers are coming and buying as they know the potential of this area.