Well it looks like the Samsung 950 Pro is on backorder everywhere, can this be installed after or is it better to wait until I can get it to complete the build.....assuming I have all the other components.
The idea of getting the 950 would be to install it and utilize its speed and performance for your Windows install. Without it, you would be waiting and doing anything else with the 950 (using it as just storage for example) doesn't make sense..then your really paying for a premium solution and using it for the wrong purpose.
The reason its out of stock is due to the imminent release of its successor the 960PRO & 960EVO. The newer devices will be faster but they will most likely carry a premium price tag at launch (I believe Dec 13 is release for some variants and Jan 2 for the remainder)
You can do a couple of things, wait and see what launch prices are on the 960, or buy a cheaper/slower but still decent 2.5" SSD (example Samsung 850EVO 2.5") or the 850EVO sata based M.2 we discussed a few pages ago.
I see the 950PRO is available in the 512GB variant but they are over $400