Sorry AC.
I READ the original post.
I'm saying that he's being unrealistic wanting to put it in a crawlspace with no environmental controls.
Even a completely fanless solution in an environment like that will collect dust in megaton quantities. And you can't hard-seal the device away to avoid moisture because it'll overheat in short order. Simply dumping dessicant packets in with it won't offset the moisture issue, as it'll condense out of the air and onto every surface.
Sure, if he wanted to forego his next automotive purchase, he could probably find (and afford) an environmentally hardened unit that would do MOST of what he wants.
His other option would be to convert the crawlspace into a semi-finished, insulated, environmentally controlled room. However there may be monetary or logistical problems there.
But, in the long run, he's better off building an silent or near-silent HTPC and putting it in an environmentally controlled area. Either integrated into his entertainment center, or tucked away in a closet with adequate venting.