Help with older mac pro upgrade!

I am currently looking to purchase a 2010-2012 Mac Pro. I have very little experience working with Macs but I have built several PCs over the years. I am trying to keep the costs relatively reasonable and thats why I would like to go with the older Pros. Any assistance or resources would be greatly appreciated!

1. I will primarily be using it for Adobe Suite, but I am a PC gamer as well (Battlefield 4, Dota 2, Overwatch, CS GO).
2. I am aware that some logic boards have 2 CPU slots. Can I get by with a single Xeon CPU? A. What type of CPUs are recommended?
3. What type of GPUs are compatible? I am currently looking at GTX 980s.
4. Are there any reputable websites that sell Mac memory?
5. What year (model) Mac Pros are recommended?