I STRONGLY suggest a drainage layer. Clay aquaponics balls, leica, a sheet of Matalla pond filter, or gravel - at least one inch, covered with regular fiber glass screening. The growing substrate that goes on top of that needs to drain and breathe, otherwise you will end up with a mucky anaerobic mess.
In my experience, the plants do ok under the heater if there is enough space, and the heat is not too high. I doubt the UTH will do much for you from beneath the floor, esp with Melamine (is this why you are worried about water damage? ... Honestly, I don't see that material as a good choice for bio-active. I expect water will get in and swell the particle board somewhere.) The UTH also won't be able to heat through the drainage layer up into the upper substrate if there is the proper amount of aeration/fluffiness in the media.
I wouldn't bother with ordering the buggy clean up crew. They are better suited to the minuscule droppings of dart frogs. A bit of home compost will have sufficient bacteria to break down incidental small stuff, and you should clean any waste piles as you find them. Fertilizer can be fishwater, or a compost tea. You aren't raising tomatoes in there, the plants can grow slowly - without LED's, light is probably your limiting factor anyway.
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