1. nearly every precon is a good launching point for anyone getting into commander

2. how long will it last you depends on how well you vibe with the deck. How long till you upgrade it? Depends on how often you play with it, but thats to be expected.

3. Everything you should have: deck, sleeves, deckbox, d6. d20s arent super necessary, you can always use a life counter app or just a calculator to track life. There are also life tracking spin counters you can purchase.

Do you remember what you played, what you liked about it? I highly reccomend some research into that so you don't end up with something you don't vibe with.

Almost any precon is a good starting point! MtG has controversially been doing a lot of decks for other IPs, so it would be worth it to get a deck (or set) of your fave IP, if that's FF, that might be the move.

Decks last literally as long as you're interested in playing them. My first commander deck (a ship of theseus) is over a decade old, and I still love to play it!

For accessories, you'll need 100 sleeves, Dragonshield is my go-to, a deck box, and numerous D6s to represent counters. Life is usually measured by phone app in the modern day.

Generally I'd advise against new players jumping straight into commander. There are so many cards on the field to keep track of and, thanks to the singleton nature of the format, it'll even take a long time to get familiar with all your own cards on top. With you having some prior experience and also having had the chance to play commander with a borrowed deck (and having fun) you should be fine.

If you like FF and/or like the way some of those decks play, I don't think think that bundle would be a bad idea, definitely not a mistake. The only thing I'd mention is that a portion of the overall playerbase is quite disgruntled with all the "universes beyond" products, so if you plan on playing with strangers, you might get to experience that.

As for how long they'll last you without any upgrades, that probably depends on a few factors. If your playgroups decks are of significantly higher powerlevel, to the point where you feel like you can never do anything, you'll probably be looking to upgrade faster than if their decks are at least somewhat comparable in powerlevel. If their decks are comparable, and depending on your level of frugality, you might be able to have fun with those decks for years, especially since you'd have 4 to choose from to keep things fresh and experiment with what playpatterns you do/don't like.

As for upgrade recommendations, I personally like to think of it in two ways: There are upgrades that change what the deck can do (new/stronger combos/synergies) and upgrades that change how often the deck actually gets there. While the former feels very splashy and big impact WHEN you get there, the latter makes sure you can actually have a role in the game without everyone else being miles ahead of you. Upgrades to the land/manabase fall into that second category, and they're usually also very transferable (between decks of the same colors), so if you don't enjoy a specific deck anymore, you can just reuse the manabase it in another deck.

Lastly I'd like to bring up that usually Commander is very socially driven. People don't tend to build the best decks they can within the rules, but instead try to stay within the social norms of their playgroup. It might take you a while to get used to what is and isn't okay with your friends. Unless they also completely "cranked their decks to the max", you'll just stomp them once and then they won't want to play against that deck anymore if you do. This is also another point towards upgrading your manabase. Very very few people would complain about you running e.g. fetch and shock lands instead of tap lands. (don't worry about what those are for now).

I personally love playing precon decks bone stock. To me trying to make someone's deck work adds to the challenge of the game.

The last precon i upgraded was one of the 2018 decks (other than a league i ran where part of it was upgrading the deck, but it was returned to stock after the league).

I also own many, many precon decks, so i have a large choice so they do not get stale.

The Finally Fantasy decks were fun, i have only played Yshtola, and Cloud, possibly Titus.

As was mentioned by mirodir, commander as a new player does not teach you how to play magic. It teaches you how to play your deck.

I owned an FLGS for over 7 years, i saw countless new players learn playing commander and never understanding how the rest of the game works.