Whatever you think it will cost, double it :P I built a liquid cooled machine and to do it properly and safely required alot of research and design in terms of flow.
With modern coolants it is very safe as long as you have a decent maintenance routine as the coolant will ionise over time which is when leaks will cause issues.
It will keep your parts cool but you will get alot of dust and best be advised to put filters over the radiators. I cool mine with 6 1800rpm fans on 2 Black Ice rads and that sucks alot of dust as you want cool air moving over the radiators.
The other key thing is maintenance as you need to swap out coolant periodically and clean the blocks / fittings. This means you either need to design in quick disconnects or think how you intend to get the fluid out without getting it all over your parts.
Really it is very cool and I can definately say my pc was an achievement in skill and design and is very much my own. However it cost alot of money and is more high maintenance than a regular non water cooled system. I overclock as well so I have a very good reason for liquid cooling. If you don’t overclock then I’d suggest it’s probably a bit OTT for your needs.
If you really are into it as a project though then have a read on bit-tech or overclockers.uk for advice and builds. Good luck either way