I have a pair of WD TV Live boxes in my house and love the little gadgets. I've used them for four or five years and have yet to see a video file they wouldn't play so for me, transcoding is not a requirement. My media server is an HP Atom N270 netbook with 2GB RAM running Debian Unstable and I use minidlna to serve up video and audio; I also use this machine to manage network backups to an external hard drive. Works like a charm.
Although the netbook has a GUI installed I never use it, preferring to manage it over an SSH session.
If you're committed to the crawlspace thing (and it appears that you are), as others have mentioned as long as you can keep water, dust and critters out of the machine you should be fine; but with the WD box you really don't need anything fancy to act as a DLNA server. You seem to be committed to running a Windows machine as well, so I'm not gonna debate that point
I think the biggest challenge you'll have is keeping critters out of the machine while still maintaining reasonable airflow; so I'd think that a low-power box that didn't require much in the way of cooling would be the way to go; I think your biggest issue will probably be keeping bugs out of the thing.