Configure email system for convenient offline flash backup of existing gmail accounts

Would like to achieve the following: - Create backup archive of existing (gmail) accounts on local flash storage - Delete all history from gmail accounts (but all data should be preserved in backup!!! Under no circumstances should deletion of data in gmail account cause deletion in backup!!!) - Automatic "top-up" of back-up on an ongoing basis data as new mails come and go in gmail account - Set up email reading system to allow convenient access to backed up mails and attachments (should allow easy searching, sorting, filtering of mails, etc) - System should work in both Mac and Windows environments As we are not IT specialists, we need a consultant who can speak reasonable English and work online with us (using TeamViewer or similar) to assist with configuration