Same as UV sterilization, to be avoided

Some valid points have been made so far. I would add that you should work to a minimum output. I would suggest you base it on:

Maximum demand from the dialysis machine x number of machines x 1.25

So for 10 machines I would be looking at a minimum output of 16 L/min. If there is likely to be further expansion in the near future, it will be worth factoring that in.

The other point not discussed is the quality of water you want. Are you looking at AAMI or higher quality? Also are you looking to have a back-up system? Both of these aspects will make a sizeable difference in cost.

Most systems have redundancy built in, plus they are upgradeable in terms of output.

Water systems for hemodialysis application can work with any model hemodialysis machine. I have consulted on about a dozen clinic installations/new construction in the last 10 years. Mr Hannis said it well. Listen to him.
I have Gambro and Marcor(US company) water systems and both have proven to be vary reliable. Biomed is responsible for the water systems and hemodialysis machines. Had to learn a lot of plumbing skills. Both offer heat disinfectable systems which work very well and cuts way down on using those nasty disinfection chemicals.
In Canada a water system for hemodialysis application must have a medical device liscence designation. Is this required as well in the UK?