Franco, the case you've linked includes the watercooling loop, not sure if OLDPHART noticed that!
the XSPC kit included will easily outperform the h100, but will require occasional maintainance (top ups, plus a full drain and refill around once a year optimally!)
it will be easier to install, as all you need to do is mount the CPU block, but installing the H100 would only be the same plus mounting the radiator, which is essentially the same as mounting 2 fans, I'm sure you can manage
the main advantage to the XSPC kit is that it is not sealed, and therefore can be upgraded if you decide later on that you enjoy watercooling, however the pump may be slightly on the weak side for CPU+GPU, so really about all you could add is another radiator, but if you upgraded your pump to for example a DDC or D5, then you could re-use the 240mm radiator and CPU block in your future loop.
there are many cases that can be made water friendly, with varying degrees of effort, but there are 2 that for me stand out as great low budget water cases:
the bitfenix ronin:
http://www.overclockers.co.uk/showproduct.php?prodid=CA-129-BX&groupid=2362&catid=1850
I personally am running a bitfenix shinobi, which I modded to accept 2x 240mm radiators. it took me a lot of work to get the 240mm in the top and even then it's not a great solution, but with the ronin, bitfenix made it fit more natively (still a bit tight for radiator thickness, but the 30mm one in the Kit you linked would fit fine, as would an h100)
also, I would say have a look at the fractal designs ARC midi (R2), but I can't link you to that, as OCUK don't stock it!
P.S: here is the link to the watercooler kit that is included in the pre-built you were looking at, just in case you wanted to pair it with another case!
http://www.overclockers.co.uk/showproduct.php?prodid=WC-265-XS&groupid=962&catid=1532
but if you went for that, you would have to fully assemble the watercooler yourself (or ring ocuk and ask them for a price as a pre-built with a case you prefer!)