You should be looking at the 980ti, a very different beast to the 980. They are around £110 on ebay, get one with a dual or triple-fan cooler, not the reference single fan design.
In the days of the 970 (and current AMD lol) the nvidia/amd reference cards came first, they had crappy blower style coolers, hot and loud. So the people jumping on the new card sales, a lot went with water cooling to mitigate the hot (or suboptimal) and loud cooler. But people were doing it on the top card at the time the 980. it made no sense really to get a 970 and water cool it with a block, just spend the money on a better card to begin with.
It was sensible for 980 owners as it gave more perf on the best card, and got rid of the crappy cooler. Contrast with the 20xx cards shipped with decent dual fan coolers from the getgo, as will the new 30xx series, the coolers these days don't need fixing. The gfx card forum isn't full of people going water on the new cards, reviews pending on the new cooler but it should be decent cooling and not noisy, water just doesn't make sense.
I run a 970 in my second PC, so not hating the 970, but at this point your polishing a turd water cooling it, it's 6 years old. Less so if you are doing it ghetto with bits you have already, but if your spending money buying a block etc well they tend to be card specific. Will be hard to move on and get money back on.