FG, you could get 4x8 sheets of foam isullation at Home Depot or Lowes...

If going DWC are you gonna recirc the buckets with a res...

The tds pen tells you the ppm of the salts or how much electricity can be conducted in the nutrient solution...As for how many ppms to use at any given stage of plant growth, is dependent on the specific strain..So I'd say just start off lower than what the bottle says and go up gradually with the nutes, watch the plants let them tell you if their starving, give em more. If their burning then your given them too much...

Experience with the strain your growing is the only way you'll find that sweet spot for how much nutes to give that given strain...Just remember to keep records of what you gave each different strain you grow for future reference

sorry for the long ramble..hope this helps

D