Just make sure you have at least i5 for your CPU, 8G of ram and GTX 770 for your GPU and the rest is not important.
I disagree. A good monitor and a PSU are your best investments.
SSD!I would also stay away from laptops as development environments. Just use a gaming PC and you're set. Modern development tools don't hit your system as they used to.
Honestly, I would never buy anything more than 13". If it is to be portable, it must be portable. Not something "portable" because someone put a "portable" sticker on it.