The motherboard is a B85 series and should suffice... within reason. The build quality of the boards used in the Dell computers is not as good as you will find in aftermarket boards. I have had a PCI-e buss literally come off in my hand. Also, the release on the PCI-e buss will be occluded by the card, so you will have a dickens of a time removing it, if you ever have to.
Definitely upgrade the power supply. The stock case has size issues with larger GPU's. A single fan unit should be fine. Warning: If you change the motherboard, you will most likely have to by a new copy of your Windows OS.
Where to buy is a personal preference. Dell will not sell you these things. Newegg, Microcenter or amazon should do. I use...