Hi There,
Yes you would idealy need a polishing unit, whilst 002-006 is usable what a polishing unit will do is take out any peakes in the quailty, for instance on any RO unit when first used the first litre or so will always be at higher rate of TDS but when it has been running for a short while will drop to its lowest reading. The average though on a 250 litre tank is 002-006 which you could probably get away with, but for the sake of a a little more money....
GE Merlin say that the membranes have a life of 2-3 years, with no back flushing. The pre-filter on the merlin would need changing about every 10-20,000 litres of product water.
ALex