Fully understandable that your circumstances limit budget, But if 400 is your budget, you need to make compromises. You can't have both specs and low price.
Your Requirements Recap
Budget: 300$
Print Size: 220x220mm to 300x300mm
Materials: ABS, PLA, PETG (optional: Nylon, Carbon Fiber)
Features:
Reliable, low maintenance
Good firmware
Cheap/easy parts
Good print quality
Full auto bed leveling
Long-term use (3-4 years)
If you go with
bambu lab a1
ender v3 ke
Creality Ender 3 V3
Creality Ender 3 V3 Core XZ
Sovol SV06 ACE
You will have to make compromises, like ABS is very hard to print for beginners on a open print bed. A enclosed printer like the P1S or anycubic kobra s1 is much more helpful and its even possible to print certain nylon's brands.
Building an enclosure for the printer can have you end up spending the same amount instead of just building a enclosed printer.
If your budget is right now $400.
You need to be willing to at least way the pro's and con's of price vs specs.
anycubic kobra s1 software wise isnt as great as the bambu machines
But you need to do research and figure out which is worth it.
Or maybe save up more money?