These seem to be a very valuable tool, just a bit pricey for all the machines you need to get. You might be able to buy one unit and read multiple parameters given they use the same wavelength of light. (470nm or 555nm seems to be the most popular) Probably the most accurate method for testing after calibration with their known solutions.
The only thing is that many people who are using the different fert. methods on this forum have stopped testing altogether and just do frequent water changes to decrease nutrient load.