The idea that a laptop is not suitable for this task is not fully accurate - there are laptops that will perform quite well. The thing you have to keep in mind though is that a comparable desktop will be about half the price.
In either case you do not need to build it yourself, unless you want to :).
As VFXM posted, a Sager will work well, but I owned one and they are very heavy. Ultimately I would ask why you are getting a laptop if it is going to be 7+ lbs, because it won’t be very portable.
A couple of more portable ones that will work well are:
MSI GS60 Ghost - 16GB Ram, Quad Core processor, GTX 860M GPU, and only 4.36 lbs. http://usgaming.msi.com/Product/2014/Notebook/GS60-Ghost/
If you are willing to wait a couple months, Lenovo is releasing the Y50, which has very comparable specs to the MSI GS60 but it also comes with a touch screen. The touch screen does add a pound, so keep that in mind.
I personally will be buying the Y50, but if I did not want touch, I would get the ghost.