Another possibility to improve reliability with SD cards is to use a distro that is designed to not make any writes to the storage in normal operation. I have some NTP servers running on OpenWRT and they work great.
Along a similar vein (if sticking with Raspbian, et al distros is important), you can e.g. disable swap and journaling, using ramdisk for logs, etc. as well as keeping lots of free space available for wear-leveling and a few other ideas: https://raspberrypi.stackexchange.com/questions/169/how-can-...