Ok lads and ladies, I've been giving this some thought now. I can't
afford a new server machine, and probably shouldn't be running this
monster 24/7. Digging around, in my price range is a Linksys NSLU2
(about £60..) which can be reflashed with Debian.
I run a NSLU2 for my main internet facing machine at home, installed
using two flash discs (1GB & 2GB). It functions as a mail gateway
(postfix) relaying to my other server, handles internal DNS and DHCP
(dnsmasq). I use dropbear for SSH rather than openssh and run it on a
non standard port, I also run fail2ban on it, and munin monitoring
(although this is hacked so its started by cron at the appropriate times
rather than consuming precious memory constantly). It is probably
slightly overloaded, I intend to purchase another to offload some tasks.
But it handles all this fairly nicely, aptitude is a bit painful to use
but it does the job eventually.