Water leaking on stuff depends on how well you put it together and what components you used. All Dangerden would be top knotch stuff not at all prone to leaking although of course 'shyte can happen' but you don't really hear about it all the time. Every once in a while though you'll hear of someone exclaiming about theres leaking and it is indeed a tragedy
I've known a few people though who's had their systems leak and using some isopropyl alcohol to clean it, letting it dry a day, it still worked after words. Especially if you use Distilled water and about 10% or less of zerex
You don't have to refill water per se, but it's recommended that every 6 months you drain and refill the whole system due to water loss, but just the fact that it can still get dirty. Even if it doesn't get dirty, it's best just to do that kind of annual maintenance to make sure everything is on the up and up.
As far as temps concerned, you can't really give an exact number on that.
Depends on what temps you have at stock cooling, or if you have after market or what. Load temps would be lower than idle temps since it can get the heat away quicker. In terms of graphics cards it can easily cool the GPU up to 20C lower than with it's normal heatsink.
Noise of course is very minimal, but when compared to a stock setup it might be the same unless you had a lot of case fans.
If you went with a weapon core for the heatercore (the radiator, but a heatercore and rad are a bit different) it has 2 120mm fans, so if you have 2 of those, run them and you will be able to tell approx how loud that'd be. Those would basically be the only noise, you wouldn't hear the pump at all over those.