There is a possibility of some overheating, however the enclosure I bought has a fan in it.
If you want a cheaper option, you can buy an Internal 3.5" SATA HD, get an External Enclosure with Internal SATA connector and External Firewire and USB. Thats what I did.
Cheaper option would be to go Internal IDE and just External USB.
Because buying an External HD with the drive already in it is usally more expensive, IMO.
Newegg's search option for external enclosures yields alot of choices.
Search Configuration Here
Hope It helps