I've been looking at this a lot recently, and unfortunately, there is no direct answer. There are budget and kit options, but the experienced don't generally recommend them.
The first thing I realised was, that to water cool my system, my case was going to be tight, so I factored in a new case to begin with. When you start investigating cases, and the water cooled builds in the picture thread, that'll give you an idea of what you can do. Then, price up the individual parts. Waterblock (or blocks), pump (best to go as powerful as possible, or risk having to upgrade later), reservoir (can be combined with the pump as a "top"), radiator, and fans.
HTH, and there are some very helpful guys on here that'll point you in the right direction. I've found the build logs to be really great.