I'll tell you know, straight from experience .. The Kandalf isn't a good enough case for custom watercooling. It's big enough but the design is poor, even for modding. The res would have to be external.
As a PC tech, I wouldn't ever offer water cooling as a solution or service as 1. It would take too long and thus expensive and two, it would be difficult to offer any form of gauaruntee and 3.. I'm not sure if my public liability insurance would cover any accidents as a result of faults.
It sound to me if he is trying to sell you something just to make some fast cash.. However, he maybe underestimating the task in hand.