you dont even need to buy a caddy you can just buy the USB to IDE converter cable/IC without a enclosure. both are very cheap around the £20 mark
The USB adapter with power adapter is about £10 on eBay, including shipping. Those work great, i have 2 and will buy more.
http://www.sabrent.com/products/specs/SBT-UI235.htm is about minimum and it handles 2.5" drives too. You can also use it to make an internal drive a USB drive if you have an USB port left on the mbo. These enclosures may be cheaper http://www.masscool.com/enclosures.asp. I could never make a bootable USB drive with either of the two brands, but that also depends on the BIOS (my youngest is two years old).