Re: Is it just me or does this seem like a lot of effort and risk?
If you stick with the generic OpenBSD kernel, too, you can probably get away with building this on an old machine, and then if you want to replace it, popping the drive into a new system and not having to do much more than tweak the settings for the network interface, which may have a different name depending on chipsets.
Indeed. I'd build this with Linux rather than OpenBSD - but that's more about familliarity than anything else. The two are pretty much equivalent.
The box that runs my business mail started out as an IBM Aptiva - 400MHz? Something like that. It was an old dog that I repurposed to try out a home server.
That personality now has none of the original hardware - it's now an Athlon64 (still running in 32-bit mode, though, because it's evolved from the Aptiva installation) running on a 2005 Winfast motherboard. Moving from one chassis to another is trivial in Linux (as it is in *BSD, IME).