You could even go with a dedicated PC with a package like m0n0wall or PfSense. From my experience, an old P4 2.6 gHz with 256 MB RAM comfortably routes at 100/100 with PfSense.
The interface is quite intuitive and of course, there is the option to SSH in and get a commandline.
Also, from what I read, load-balancing or failover is quite easy to set up, so reliability (because of old or non-server-grade hardware) might not be as much of an issue as it would seem. You can also backup the configuration quite easily so if you have some identical boxes, replacement shouldn't be a problem at all.