Oil filled radiator it doesn't matter how many fins if the price is right as they have a thermostat on but always read reviews before buying., As to PPM buy a blue labs truncheon, always buy a decent pH pen, cheapi ones can help you ruin your grow, like the guy said above use pH test kit drops till ya can get a good pen.
BTW if your going to use tap water let it stand for at least 24 hours ( this is NOT bro science) I read you are getting organic soil, use tap water if you can, a garden butt is great.