Hi, first post 
from my limited knowledge/experience i'm impressed with the quality of water claimed to be produced by the Merlin RO system. The time factor seems to be a big downside.
I have a friend in North Wales with a water pressure of 100 psi plus and a tds similar to your approx 60ppm, his twin RO system from Facelift systems fills his 650 litre tank from empty in roughly 2 hours. A tds reading of the tank shows 0ppm, he hasn't had to renew any of the two DI resin cannisters on his two man system in a year. His tank out put runs through the DI as required by the pumps. I have no idea exactly what membranes he has in it, Facelift systems could probably tell you.
Myself i got a HF5 40/40 kit from Gaps water for approx £600, (single RO) i have a water pressure of approx 40 PSI and a tds reading of 130 ppm, my product water was coming out at 30 ppm and taking me 3 hours to fill 320 litres(half a tank). I bought a booster pump from ebay(£120 with postage) with a built in pressure tank and i fill from a header tank at 80 psi and i have not improved my TDS, my fill time now for an empty 650 litre tank is approx just over 2 hours but my TDS has not come down.
The best priced 40/40 membranes i have seen now is, suprisingly from Ionics who do the HF5 low pressure membrane and (champ)housing for £300 i havn't seen it cheaper.
Hope that helps you ! 
edit to add, if you're concerned about waste water you would be best to go the twin RO system route.
Hiya mike....
My tap psi was 8psi & my tap tds was/is 550+ & i was getting 34ppm from my hf5 40/40 without a pump & @ only 8psi mate
summit dont sound right there man
Now i'v added that pump im getting 14ppm from a massive 550+ppm....
I think your ro might be under working man.......
Just my 2p worth.....
Take care & kindest regards
Justin