Setting up everything yourself is a pain in the butt. IMHO, it's not worth your time. I'd look into a turn-key solution instead. Zimbra has already been mentioned, but it is _very_ heavy on the resources and more or less requires a dedicated Linux box (or VPS). I would suggest you take a look at Atmail. They're a new comer, but it is rather promising. It is using all 'the usual suspects' but without you having to configure them all by hand. That said, setting up your own email server is great way to learn about your system and email in general, but you should be ready to spend some serious time on configuring everything.

I've given up hope on those cheap routers. Sure, DD-WRT and Tomato are decent products, but they don't come close to a box with pfSense. Just pick up the smallest, cheapest and least power consuming ITX box you can find and install pfSense on it. You can control it all from the web browser. Best of all, it's based on FreeBSD.

If you're able to get a hold of it, IBM OmniFind Yahoo Edition would do the trick. Unfortunately Yahoo pulled the plug when they went into bed with Microsoft (Bing). I'm using it on a local intranet, and it works great. If you have a deep wallet, you can always look into the commercial version IBM offers, but it is really nothing but a packaged version of Apache Lucene.