Oh, and this jackass is lying to you too. Steam versions of FFs 4, 5, and 6 are cancer. Avoid them, just use emulators. I played 5 on the GBA and enjoyed it, it has some postgame stuff that can literally make a character immortal to everything except Atomic Ray and Ultima if you still want more job system in your life.
1:
It's on every console and handheld since the NES, but just… don't play the NES version. The EXP and gold curves are kind of fucked and it takes forever to grind. Play the GBA or PSP versions.
2:
Don't force yourself to play it if you don't find it fun. Usually comes in a two-pack with 1 because they know if they sold it on its own no one would buy it, so play it on the same thing you played 1 on. If you're familiar with the progression systems SaGa uses, this is where it came from.
3:
Just don't. It takes a necessary half-step toward making the job system fun to use but punishes you for levelling too much too early, is hard as balls, requires grinding, and has some real pacing issues, including a final dungeon that is like four hours with no save points.
4:
I don't know, because every time I try to finish it I make it to the moon and put it down entirely. It's one of my least-favorite FFs and Kain is a shitty character. The original has a cut-down script, the GBA version lets all the dramatic sacrifices come back to life and join your party, the DS/PSP versions have extra content, longer scripts, and let you assign the abilities of the dead party members to your still-living ones.
5:
GBA has a better translation and more content.
6:
SNES has a more fun translation, even if it deviates from the Japanese quite a bit. It also has better sound quality on its excellent OST. GBA version adds some content that was dummied out, but the sound is a bit worse. Translation is still alright, and they fixed some of the major bugs from the SNES version; those bugs were not game-breaking as much as they were weird calculations that could be exploited. This is the one I recommend starting with if you're not sure which to play.
7, 8, 9:
Emulate PSX versions.
10:
From what I understand the HD rereleases were fairly okay, because they were more or less straight ports and texture/model updates. Otherwise, emulate PS2 version.
12:
The Zodiac Age if you have a PS4, otherwise emulate International Zodiac Job Version with the English patch. The original is still fine too, but once you have that L2 double speed button it's hard to go back. Or just wait because a PC port is pretty much inevitable at this point.
13:
I don't know.