Don't buy from Best Buy or any other "big box" store. Buy from a specialist such as pro-tools-pc.com. They build PC's for Pro Tools, so they use the right parts for performance and compatibility. Minimum specs for me would be:
intel i7 6 core(6800K or better)
32GB of RAM(more is better, but 32 seems to be the "sweet spot")
500GB SSD for system drive(yes, people use smaller, but by the time all my VI's are installed, even with samples on separate drives, my system drive ends up a bit over 300GB)
Choose a graphics card to cover your needs(I need 3 screens so I got a card that can handle that. I ran 2 graphics cards in my previous rig).
650 watt PSU would be my dead minimum(running 850 watt now)
As for OS, as much as I preferred Win 7, I am running 10 now(with a little teeth grinding
). Overall, Win10 is fine(and since you are coming from the world of Apple, your learning curve will be much the same anyway).
Re the second drive: yes, you NEED a second drive, and you want a 3rd drive(if you have large sample libraries).
If I read correctly that your budget is only $1K, then you should be looking at a used HP Z800 workstation with dual 6-core XEON's like this
http://www.ebay.com/itm/HP-Z800-Work...4AAOSwH09ZFLsY