1) The inbox seems to be limited to one thousand or so messages, but other boxes created in IMAP don't have this limitation. Max disk space is configured via quotas, so this is only limited to your disk space (500 or so gigs)
2) There's a referral system:
3) Terms of Service limit disk space to domain-hosted services, such as email, databases, and website content. So, no, you can't just store whatever files you like.
4) Dunno, but why would you want to? Dreamhost hasn't always been clear about how many accounts you can have, but if it's legit, the don't seem to mind. The Panel appears to support multiple accounts for a single user.
5) The panel does not make it appear that it can remember more than one credit card. Same goes for billing address(es). I can't blame them, as this would appear to be a suspicious setup. Some one person needs to be responsible for the account.