Purified is usually filtered one way or another, possibly RO.
TDS stands for total dissolved solids, not salts. A TDS meter and a PH meter are the two tools you really want for hydro.
Water quality is important but...I use some pretty horrible tap water and have nothing but good to excellent output, so...
Salts shouldn't be a problem if you change your res/nutes often enough or keep a steady influx of new solution.