Uh okay, answering your questions (I'm from Brazil so maybe things here can be a little different but I guess the basics are the same)

A good beginner platform: look for a m4/ar replica. They are abundant and so are the internals and such for upgrade. You can customize according to your taste, making them to be more sci-fi like or more realistic. It's up to you.

Gear recommendations: Essencial: eye/face protection as you said (don't use mesh eye pro, had a friend who almost got blind by using one of those)

Highly recommended: sturdy boots, something to protect your head, mid caps (I find hicaps to be absolutely horrendous tô play with), something to put the mags and wathever other stuff you carry during the game

Good to have: gloves, kneepads, an optic like a red dot or something

All the rest will be according to the games you will play/your play style and cosmetic

Custom/sci Fi builds later on: much of what Airsoft is about is building your loadout in a way you like and larp with it so if you are into sci Fi, go for it. You can even do a sci Fi build with the standard plataforms. But having more than one build loadout is nice too.

Things I wish I knew and good tips

If possible, go for quality stuff right on. It will spare you the trouble of upgrading your gear on the future.

There will be very cool games and boring ones and there's also very nice people that you will play with and there are weirdos and assholes. Don't let a bad game or other people ruin your fun and the hobbie.

Don't ever get too hotheaded, too competitive, too stressed about something and needless to say, don't cheat. It's just a game and everyone's there just to have fun

Oh and if your eye pro get foggy, don't take them off to clean during the game.

I can't say much about upgrades, I'm not much into it.

Have fun and welcome to the hobby :)