I would recommend to ask this question on the dedicated developer list for Overpass API:
API to get some expert-level feedback:
http://listes.openstreetmap.fr/wws/info/overpass
When posting your question, could you please also include some pointers to the installation documentation you've tried out before. Also, please include further details like:
* do you need current data only or also attic data (=previous object versions).
* How about meta data for objects, is this also required for your use case?
* Do you need continuous updates or a one-time set up of the db only?
* Do you want to import all objects or only certain types (e.g. only highway=*, etc.)
Roland (developer of Overpass API) has recently started to integrate db compression features. However, this is not officially released yet. The Overpass API development list archive has very detailed steps on how to set this up. This might also help to save some disk space.