DON'T get the Steam version, get the retail version and install it normally - don't activate it in Steam. I wouldn't recommend eBay unless it's certified as a brand new version. You can buy a disc copy from amazon for a fiver, this is probably your best option.
Steam does not allow the exe to be patched, preventing you from using the hydrophone fix etc. Retail disc install allows this, and is a lot cheaper too.
