Carbon postfilter is supposed to remove chlorine I think. It's not essential for w/c water (only if you're drinking the product water).
I have used a merlin to prefilter my water for about 4 years now. I recently moved from a soft to a hard water area (275 ppm from mains). I saw the PPM of my output water increase from less than 10 to around 110. This meant my polishing DI resin was only lasting 6-8 weeks, as opposed to 6 months in the soft water area. If your 'raw' tap water is as high as you say (300+ ppm) you will encounter this problem, also, the lifespan of the expensive membranes in the merlin will be greatly reduced.
My suggestion is to add a small resin bottle containing water softening resin to the feed line between the mains and the merlin unit, this will dramatically reduce the input TDS. I did this and managed to get my input TDS down to 140, giving product water around 60ppm TDS. Softener resin is much cheaper to replace than DI resin, so over the long term you'll save money.
Hope this helps