Free Agent is a small download and is grunty enough to cope with your ISP's 30,000+ newsgroups. It has all the bells and whistles to decode attachments, keep a reasonable disk cache over time and a nicely configurable display. It will allow you to permanently keep articles on disk, and will do several message fetches at once.
If you can be bothered downloading headers and then marking what you want to fetch offline, you can easily use Free Agent in offline mode. Here in Oz we have fixed price local calls, so it doesn't really save you money to keep logging in all the time, so I tend to read news while connected.
It is not particularly HTML aware but it will allow you to copy a URL so that you can paste it into Opera. This is all you really want, no need for bloated features. Free Agent is fast to load and small.
Agent is no longer under development, sadly but you ought to be able to find everything that you need in version 1.11.