Hi Hari,
Addressing the dial-up cost issue, in OE it is possible to copy or export
downloaded messages to another directory. You could, perhaps, do this once a
month. A potential problem with this approach is that the ability
automatically to group messages by thread is lost.
I am sure that it would be feasible to set up a database application to hold
the downloaded messages and provide the additional features you require.
However, being spoilt by (relatively) good internet access and the Google
Groups online database, I have never felt tempted to pursue the disk
database approach. Of course, were internet access to become contentious,
either in terms of cost or availabilty, I would review my approach.