Hi !!
For the signature, there's already a database field prepared within the contacts table. I'm not sure what you have in mind for the ".sig file" but a simple text signature would be good to start with. Please discuss additional features (like attaching a VCard as signature) here at the list.
It would be great to think about multiple per-user signatures that could be related to entries in the addressbook, i.e, many users would like to have an english signature for english people, spanish signature for spanish people, company signature with disclaimer for work related mails or a funny signature for friends. Signature could be selectable through a combo box when composing the message (if there is more than one signature). If this sounds good i could help coding.