I noticed several things in your build that differed from the link: cheaper PSU / CPU cooler; more expensive RAM 16 vs 8gb, more expensive video card.
Yeah I realized 16GB of RAM would be useless in a gaming system so I altered accordingly.
That build is fine but I do not touch mATX on full desktop systems because then you limit your expansion options which shortens the life of your system. I really don't recommend motherboards over $150 because you'll most likely never use all the features.
If you have any extra money leftover I always recommend putting that in getting the best GPU you can get because on a gaming build that make or break the build's performance far more than a CPU upgrade or anything else can.
The PSU is the same one I use only it's the 750W model and I've had no complaints. It's actually made by Seasonic which makes really great PSUs.
Additional question: If you had to budget a monitor, what monitor would you suggest.
Monitors are a lot of personal preference which is why I typically don't include them in builds (same with keyboard / mouse as I'm not a fan of blowing hundreds on them). I'm using an Acer 24" 1080p that has served me quite well over the last couple of years.