I'm running a small home NAS on OpenBSD, in a very similar configuration as your intended configuration, right down to the rsync backup scripts. It's worked very well so far, though I've only had it in place for a bit over a year.
I chose OpenBSD over FreeBSD due to being far more comfortable with OpenBSD than FreeBSD, and my desire to run my favorite OS in a meaningful way. I say go for it, and (like all systems) make sure your backups are good (that means test them periodically). -- Jon Tabor tab...@obsolete.site "I desire peace. I also *require* freedom. Not only for myself, but for Earth. If it can be achieved through peaceful ends, wonderful. War is waste. However, the only thing worse than war is the loss of liberty." -John Ringo, _The Hot Gate_