Necessary meters? Alternatives?

Hi guys. I'm getting ready for my first hydro grow and I have some questions about the meters required to monitor the nutrients and what not.

What are the most essential, or absolutely necessary parameters that need to be measured? I'm guessing pH and EC. So, I believe I'll be buying a pH pen, probably off of ebay. As for the EC, this might sound funny, but would I be able to use an electrical meter, say a DVM or multimeter to measure the EC of the nutrient solution? As in a corresponding resistance or something?

Also, I notice the EC truncheons show a scale of both EC and ppm, so would I also need to get a TDS meter in addition to the EC?

In short, could I get by with a pH pen, EC truncheon, and thermometer and still achieve optimum results? This will be a small 400w setup. Any advice is greatly appreciated, thanks.