Where can I buy Welcome to the Game?
Just install Steam, make an account, and go to this link. It's 2 US dollars (a few cents cheaper on sales), and basically any remotely decent laptop/computer can run it well since it's a very simplistic and small file.
How graphic is Welcome to the Game?
There's fairly disturbing gore, imagery, and nudity, although a nudity filter can be turned on in the options.
What exactly is the Deep Web?
The deep web is a section of the world wide web not visible or accessible on conventional browsers. That's all. It's not some nightmarish hell on Earth, it's just a massive chunk of the internet slightly harder to access than usual, which is why sickos tend to spot it with disturbing/graphic content. This content can sometimes be absolutely horrific, however, like actual child pornography and snuff films, and shouldn't be taken lightly.
Does Welcome to the Game provide an accurate interpretation of the deep web?
No.
The basic process of Deep Web navigation is handled accurately. You have to install a specific browser (A.N.N) and use the browser to open up a wiki, a list of random websites. Like in the game, being on the deep web paints a target on you in the eyes of IP tracers. This is why you should always go into the deep web with multiple VPNs protecting you, because constantly resetting the router like in the game is a needlessly tedious tactic. Of course, you're not going to get your home invaded by serial killers or kidnappers like in Welcome to the Game, and all "documented" cases of so are just fictitious creepypastas. While the depraved content you see in the game have real-world counterparts, these counterparts are almost always hoaxes intentionally made to fearmonger and strengthen the deep web's extremely exaggerated reputation. As mentioned above, however, the real deep web still contains some cases of genuinely repulsive and inhuman content.
TL;DR - The interpretation of the deep web in the game is a combination of real material and extremely exaggerated material (mostly the latter) for the sake of horror and entertainment.