Thinkpad x300 HDD Installation

[abz155]

Hi,
I was wondering if anyone could give me some advise on installing the hard disk in a Thinkpad x300
The laptop came without a hard disk or fittings and also no HDD cover.
All I have is the laptop with a slot on the left side.
Would I need to buy the caddy also? I have read somewhere that the caddy is not needed, just the rails.
I'm a bit confused since I don't have any other hard disks around to test with.
I have found a hard disk on ebay that fits Thinkpads x300 series.
I'm not sure what else to get.
Any info is much appreciated, Thanks a lot!