We can purchase solution - it does not have to be free (and in fact would probably want to purchase since most of the free solutions I've found that do this have significant limitations).
Already moved all of the MBOX files/folders to a PC, so solution can be software that is installed on PC or Mac at this point.
Looking for a solution that will take all of this email (keeping all metadata and attachments intact and keeping Apple/Mac Mail folder structure intact) and convert to 365 or PST. We need to ultimately move this to Office 365, although we can convert to PST first and then upload those PST's to Office 365 if necessary. We have a client with 20-50 GB of email data stored locally in Apple/Mac Mail on their Mac.