As explained many times before, group list is for XPAT server-side searching (if your USP supports it).
I suggest that you trial Newsleecher or Newsbin for a month and then come back and report once you've had the experience of trawling raw headers and storing them locally to a satisfactory level of retention.
To give you an indication, I currently store 5 groups for 800 days, and that's 20GB just for the heavily-filtered headers I choose to keep. You can easily pull a few GB a day of headers only, 95% of which is useless spam, viruses or Dutch
Not to mention how often you can count on your db corrupting and having to start again.
This is why anyone with any sense lets the indexers do the heavy lifting with today's usenet.