Thunderbird is a good choice, particularly if you're handling multiple email accounts (something Outlook has been a little awkard about, historically). Thunderbird will not be able to connect to an Exchange server, so could be less useful in a corporate context.
Re Outlook's data files, you might find
>this discussion< of interest. Thuderbird can import some, but not all of Outlook's data. It can import contact information, but does not have all the same fields as Outlook, so there is a possibility of some data loss on the way (e.g. multiple phone numbers/email accounts for one contact).
For transferring over email, including sent folders, etc., I strongly recommend switching to an IMAP email account with your ISP, if possible. (That facility may already be there, you just don't know about it.) IMAP is an advanced email system that is good if you plan to access your email from multiple locations/computers. Basically the email is kept on the server; you just download copies. IMAP tracks which messages have been seen, deleted, etc.
The advantage for you is that using the old Outlook, you could copy all of your email into the IMAP account (drag and drop; nested folders are possible with IMAP), then it will all be there when you come to connect with your new setup.
