Marine depot has the Spectrapure 90gpd for$179.99 and with my 10% coupon it is $162. Add a gauge and a flush kit and that might be the one.
They are always my recommendation, making RODI water isn't the most complicated thing in the world, but they are true water treatment experts. With 40tds I'm sure you'd get long life and great performance out of any decent brand
My source water is around 75tds, I got 3+ years out of the first set of filters and changed the membrane just to freshen everything up.