Yes you can see the prices in real time as they are ("live") on pcpartpicker.
Win 7 is fine but Win 8.1 is better overall, honestly. Just use classic shell. Looks and feels like Win 7. Plus with a win 8.1 license you'll get a free upgrade to a win 10 license when that comes to market.
Even if you don't use classic shell, and I suggest that you do since it makes Win 8 so much friendlier, Win 8.1 is just a better operating system than 7 is. Win 10 is looking even better, running like Win 7 with all the features of Win 8 presented in a less in your face and inconsiderate manner. Win 7 is old. You really shouldn't get it. It won't even support DX12 when it comes out.