(Scroll to the bottom to get my overall opinion, but as a summary: could work, but there are better offerings out there)
Roundup is a simple, clean, python-based issue tracker.
http://roundup.sourceforge.net/
The project seems active / healthy. Commits are still being made at least, and developers seem to respond to inquiries on the mailing lists. Unfortunately the MLs are SF-based, which are always hell to use.
"I" installed a local roundup to test.
Searching was fast; I managed to get a search to take about 5 seconds, but that was the max. The search interface was perhaps a bit overcomplicated, but worked well enough with the defaults. One neat feature is that you can group bugs by an arbitrary parameter; other systems force you to only look at a bug of one particular parameter type at a time (e.g. "all new bugs", "all open bugs", etc.).
Roundup supports custom fields, but I'd say it's painful. You have to write the HTML to match your custom DB schema. Ugh.
Once setup, there's a nice interface for adding options/values (i.e. rows in the tables) to each field.
I'm not quite sure. My package manager did everything. It was literally about 15 minutes between "how I can get roundup on this system?" to "oh look, that's how I add my own custom priority".
Everything so far, Roundup included, supports emailing bug reports, which is the one feature I'd really like / would use.
Another nice feature is a CLI and ease of scriptability. Doing things like emailing a list of new bugs once/week sounds simple to do for the unix- and python-savvy.
I think Roundup could work. I wouldn't want to be the one to initially configure all of its fields / html, but after we do that once it should be good to go.
Comparing the trackers I've personally looked at, I'd rather see us with Roundup over Bugzilla, but not over Redmine.