I've been wanting to start a new game since forever, but there's something I wanted to be sure of. What difficulty should I select for a first game.

I did heard (mainly through the article of Hardcore Gaming 101) that the difficulty setting also influences your accuracy with guns along with your enemies. So the article recommended to start a new game (even if it's your first one) on Master so that your accuracy wouldn't suffer. But I've also heard sources saying that Stalker is a good difficulty to start on. I'm not sure what to think here.

I'd also mention that I installed the unofficial 1.0008 patch (I got it here: http://community.pcgamingwiki.com/files/file/503-stalker-shadow-of-chernobyl-unofficial-patch/), the modified LuaJIT library http://community.pcgamingwiki.com/files/file/504-stalker-soccop-xetrills-modified-luajit/) and the latest ZRP mod (since I wanted my first experience as vanilla as I could while being stable). I don't know if any of those patches changed that.

Thanks in advance.

I'd say start on the stalker difficulty and see how you go for half an hour.

The main difficulty spike for many people when playing SoC for the first time is the fact it doesnt play quite like most other FPS games.

The main difficulty isn't because enemies often outrun and trap you in circles from all sides but because bug that cause spawn in front of you upon loading game. You want be prepared for that any time, just in case but can screw your playthrough with high tier enemies and radiation. Also in some places there are enemies that will respawn because of quests that will restart every time you leave current map. If you keep that in mind it's actually fun game, so no point of speedrun or rushing that way because it's suicide playing that way because of how AI works and design of maps. The game as it is on normal equivalent of difficulty is most enjoyable but still decent amount of savescumming is unavoidable so don't rely on any autosaves placement. Depending on how you play even easy can be hard mode or hard difficulty easy so nothing is forced, game adapt to you.