After spending a huge part of last night googling around I found the Intel Entry Storage System SS4000-E which fits my requirements:
- Support RAID5
- Use normal hard disks
- Integrated stand-alone solution
- Decent air flow to ensure proper cooling for the disks (something that’s lacking in most USB2 enclosures)
The SS4000-E supports up to four 500 GB SATA-I hard disks. So if I configure it for RAID5 I would get a bit less than 1.5 TB of quite reliable storage space. I think that would be enough for the foreseeable future. The only downside is that it costs about $600 and that’s not including the disks.