Tbh it's very confusing unless you have played them all and even then it's Japanese so it's very obscure. There are plenty of videos on YouTube that explain what's going on and helpful users here. Good games if stealth is more your thing, personally I prefer straight up shooters. I am impressed with the Ground Zeroes as it was free and I don't usually play this type of game, tbh not a big fan of the genre. 5 is meant to be better graphically than ground Zeroes and as it's a full game not a prologue there will be plenty of content.

You can finish the story in 6 hours. The idea of it is that you can do the missions in almost any arrangement you want from almost limitless methods of completions.

In 5, stealth is not the only way to play. There are several videos showing mssions being completed in all out assault mode. As far as the story goes, i recommend you find an explaination of the events of MGS 3 and MGS Peace Walker, as those are the two games that take place before MGS 5. Metal Gear Timeline: 3 ->Portable Ops (not important) -> Peace Walker -> Ground Zeroes -> Phantom Pain -> MG 1 -> MG 2 -> MGS 1 -> MGS 2 -> MGS 4 -> MG Rising I can explain any questions about the series, if you have any