That don't use fans.
https://www.google.com/#q=fanless+industrial+pc finds them in the about 1K range. Any custom job will run more than that.
However I know that prices have plunges so hard that my clients don't worry about the lifespan of the PC anymore. They treat them as expendable items. You get some 499 buck machine, put it to work and replace it in 4 or so years.
I am looking into doing some work for a customer that is running 3-4x PCs in a machine shop. They are having problems with a fine oil mist mixed with fine metallic particles getting into their computers.
I have looked at industrial enclosures but they do not say anything about oil mists, just dust.
Someone has suggested a mineral oil submerged PC. I wanted something a little more tested, approved, and ready off the shelf than this. I know these have held together for years without issues but these are missing critical computers, not novelties for the company.
Size is also an issue. Any enclosure cannot be much larger than a standard desktop PC.