Advice on Buying a Used Macbook Air please [View all]

I'm buying a laptop for the first time in about 10 years. Started shopping this morning and realized there is a lot I don't know about Apple products right now.

Can the batteries be replaced? How do they fail? Fail to charge all the way?

Apple usually orphans their hardware after about 6 years (I think), so if I buy a 2020 M1 is it likely that the OS will not update beyond Sequoia or Tahoe?

Any models to avoid? or common failures to factor into the decision?

I only see two ports in the pictures -- Can I dock a Macbook to a monitor and a real keyboard?

Thank you!