Not sure where you are from but generally speaking, water cooling will be a more expensive proposition than air... whats you budget? :)
Heres an Australian site that deals in WC;
http://www.chilledpc.com.au/
Difficulty will depend on the time you have to do this, the budget you have and your desire to see it through. With anything, you will get what you pay for and with WC, quality is paramount.
Research: All I can suggest is google water cooling. There are a ton of WC forums - trawl them and note the sigs from the OPs. These will give you a good indication on what works - people dont boost about failure :)
I hope this helps
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