A Unity ID allows you to buy and/or subscribe to Unity products and services, shop in the Asset Store and participate in the Unity community.
Separate names with a comma.
Depends entirely on the size of your project. Also since you're going to play games keep in mind that many modern AAA games take up to 100+ GB of...
Yes. Only you can answer that. Mainly it depends on your budget and preferred operating system (MacOS vs Windows in this case)