Andrews right, the merlin will be no good for you, I had one at similar tds levels to you and got 035 product 60% waste, 25 ltr of resin would last me 3 months if I was lucky.
On helpful advice ( infact I think it may even have been Andrew) I got onto Gaps Water and bought an UltraTec HF5-4040 membrane. Superior in every way, I can fill my 650 ltr tank in a little over 3 hours with a tds of about 005 now and they are easier to maintain and flush, important in a hard water area with no softener.
Simon