Hmm .. besides having an old P2-400 running as a NAS on linux... i've got an actual NAS which is based on the NSLU2 running unslung connected to 2 external housings.
Housing wise, depends on the type you're after - fan / without fan and the quality.
I'd recommend the Sarotech housing for build quality but be warned - these kind of NASes are not meant for 24/7 operation. If you're after 1 dedicated NAS, go for those dedicated 1 box type eg. Snazio Tera NAS HD Pro ... these have proper 80/60mm fans that cool the hard drives efficiently.