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.
How does a good monitor benefit a programmer? Most are good enough, and the same goes for PSU's.
Get an Intel 4690k, 8GB of RAM, and a decent SSD. For the graphics card I would go with a GTX 970/980. Don't even bother looking at AMD, some of their card might look good, but their drivers(among other things) are a total mess. A 500W PSU should be enough, get one with 80+ Gold or higher and modular cables.
Case and motherboard doesn't really mather which you take. Although I would go with M-ITX form factor for those things. Bigger stuff isn't much better and will just take up space.