I think the question you need to consider is whether it's worth the time and expense. I can't address Leopard Server directly, but I did what you're talking about with Win2K Server a few years ago, going so far as to set up an AD domain with integrated DNS, DHCP, file & printer sharing, certificate services, and a RADIUS server to do 802.1x authentication with PEAP-EAP-TLS for my wireless network (which provides bulletproof industrial-strength security but renders the WLAN useless for lots of devices--I don't recommend this for home use!). I use the server to run a server-based version of Retrospect to back up the shared server files as well as all my Windows and Mac machines--which works extremely well. I also run McAfee's EPO for centralized administration of antivirus software on my Windows machines, though this has become less important as I've gradually been switching over to Macs.
While running your own server is cool and useful, looking back I have to say it probably wasn't worth the time, effort and expense to set it up. I did it originally because, at the time, I had executive oversight responsibility for IT, and I wanted to get up to speed on servers and networking so that I could talk somewhat intelligently with the IT guys. I think I ended up knowing
more than the IT guys on many subjects!
So if, like Andy, you're thinking of doing this as a learning exercise, then by all means go for it. But recognize that there will be a steep learning curve (perhaps not as steep with OS X, I don't know), and that there are probably faster, simpler and cheaper ways to do what you're trying to achieve. In fact, I probably would have dumped the server and gone with a simple NAS by now if not for Retrospect backup...I really like the scheme I've set up and can't bear to abandon it.
The other thing to consider is that, at least in the Windows world, server versions of software and utilities are far more expensive than their desktop counterparts. Consider that OS X Leopard Server alone costs $500! What I've found is that, if a desktop program costs $50-$100 (backup software, defragger, etc.), the server version will cost $300-$1000! You may want to look into the total cost of ownership for what you'll want to do before you take the plunge.
Another consideration--if you're going to put all your eggs in one basket, so to speak, by consolidating all your files, etc. on one machine, then you'll want to think about reliability and IMO that means, at a minimum, RAID-1 or RAID-5. I ended up buying an entry-level Dell PowerEdge server that now has a 1TB RAID-5 array...in your case I don't know what to tell you except that I probably wouldn't trust my life to a Mac Mini!
There are some very nice NAS boxes on the market right now that do RAID and sport some sort of Linux-based OS for setting up file sharing, shared printers, backup, etc. If I were you, I'd look seriously at this option before getting into a full-blown home server.
Anyway, I wanted to share my experience. I hope this helps you in your decision, and I wish you good luck!