It turned out that the Pi crashed because the SD card stopped working, so not the Pi's fault. So I'm still blown away.
I've listed ten of my servers that can be replaced by Raspberries.
I've decided to go with Raspbian. So I set that up on an 8gb card, and downloaded and installed: sendmail, bind9, rdate, mutt and courier-imap. Those will give me respectively: email sending/receiving, DNS, date/time setting, email reading and imap (letting other servers pick up email). When I shut down the GUI on the pi, and load all those on it, it uses 128mb, leaving 360mb free memory, which is a lot. It isn't using the swap, which is good.
I've ordered 10 more pis. Pice? To power them, I've ordered five powered USB hubs, at £2 each, and I'll put my own power supplies on them; I'm thinking of using more powerful supplies than the usual mingy 500 ma supplies they offer. I had a look at a standard PC power supply, and it'll give me 20,000 ma at 5.25 volts, perfect. Pice want 700 ma each.
From the hubs, will come USB-to-micro-USB cables, costing about 50p each. And I've ordered 10 8gb Sandisk Ultra SDHC cards, costing about £6 each. This should all arrive in a few weeks time; the pice are predicted for December 18 (they're on back order at RS), so maybe I'll see them early in 2013.