What makes the recommend video card not a good purchase? Which one would be better? As I begin picking up the components I need over time, I am finding that it would be better to have a 3 or 4GB card.
Is there something that makes the Corsair 750W power supply a bad choice? Based on the components selected the for the build, the 750W power supply is more than sufficient.
It's a good purchase, definitely in the price/performance sweet spot, but you might still want a faster card. The R9 270X is fine for 1080p 60fps at high settings in general, but it might not allow you to max out everything all the time or more easily drop below 60fps than a 280X or even 290 (which is probably beyond your budget). So, take the time to look at benchmarks and decide how much you're willing to pay for gaming performance.
As for the PSU you'll notice that for their Intel build they recommen a 450W PSU, which I think is playing it quite borderline - but a 750W quality unit would definitely be sufficient, even overkill. You'd be fine at 600W.