Pha! Ask not what is overkill, instead ask what you can kill..over...?? Ok, that sounded better in my head Anyways, point being that watercooling is only overkill if you think it is. And as many of us will tell you, overkill is very rarely a bad thing.
As to you question about the loop, if you fabbed all the block yourself, you would just need to get fittings (unless you fab those too), tubing, radiators, and a pump. A good pump will run you at least ~$50, the tubing depends on what tubing you get and how much you need, and the radiators can range anywhere from $30-$300, depending on how much cooling you want. Oh, and a reservoir, which will usually be under $50.