saving mailboxes to drive

I have a LOT of old emails relating to a former job, that I want to save somewhere safe. i.e. Not in my Mail program.

They came from a Google Mail box originally, but in Mac Mail I've sorted them into separate mailboxes, so i could file and save them, perhaps onto a flash stick, or some other external drive.

Some of the Mailboxes contain 50 emails, some over 1000, and most have attachments.

If I export one of these mailboxes to my iMac, it appears as a .mbox file.

So my questions are;-

• does an .mbox file contain all the mails safely with their attachments?
• can i import it back into Mail in the future if i need to open it (if so how, as its not obvious!).
• if i was to send one of these .mbox files to someone else, could they open it, for example on a pc?
• are .mbox files the best solution or is there a separate (inexpensive) app that will do what i want but better?!

thanks in advance

Paul