What are the chances of the unit leaking and going to town on my parts?
non, but some units do slip out with some leaks and liquid all over teh package.
Is it worth buying a closed loop or would it be better to get a "true" water cooling system.
closed loops are just a money making scheme, nothing actually spending on. Air cooling is better if you're going for an H unit...Coolermaster 212+ evo
Will i notice temp differences between this (h80) and a cooler master hyper n 512?
there are reviews available on a simple Google search, they will give you temps on the H80+hyper n512
last tip, read the watercooling sticky in my sig if you want to fully understand the real deal
, take some time and don't rush it...might even need some read throughs.