You can transfer your website, mailboxes and domains to O365 small business, no worries. Both premium and SB licensing model is for a maximum 25 user limit and allows to go with single user license.
Site mailbox is a feature which allows to store and share email and documents in project-specific folders, allowing for easier team collaboration (I don't think you tried this in the trial). This feature is not available for the small business plan and hence if you were using it, you may not be able to transfer or retain ONLY those. Hope I am clear.