I don't know how easy it would be to port a iMac or eMac to a 2u rackmount. Both the iMac and the eMac I would think have a analog video board. This would make the total pcb pieces you need to install 2. Then you would have the power supply, hdd, optical. It probably wouldn't be that bad but getting everything secure might take some thought.

Most server chassis have dust filters and usually pull air in the front and out the back.

http://www.raxxess.com/
http://www.middleatlantic.com/enclosure/knock/slim5.htm

Check newegg.com for chassis.