MTG

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).