Thanks for the input everyone, I've been considering my alternatives and I guess I'll just buck up and learn to use ports. (And a few other things...)
I looked over dspam, and while they have a really impressive web-site and their goals seem very laudable, and even in-line with the system I'd originally envisioned, I don't think I'm ready for a full solution yet. Instead, I'm going to follow Mr. Roberts' advice and try out a base system with spamd and greylisting. In the mean time, while such a system is keeping my few users afloat, I'll see if I can come up with something more tailored to our situation. Again, thanks for all the wonderful insight and advice from all responders. I'm glad you guys are here to turn to when I get really stuck on something. -Bob