I am personally an advocate of belly heat since I feel it's an excellent way to aid digestion. I have seen it done both ways, it seems to be (like many issues) a personal preference.

There are a lot of helpful videos out there where people show you how they build their racks. I watched them frequently leading up to my build to get ideas and figure out what worked best for me.

I built support into the back without covering the whole thing. It sounds like you wont have a problem with your ambients. Mine are working well in BC Canada right now .

I bought a herpstat 1 and have no complaints at all. Worth the money for sure! I have my heat tape running in parallel and all connected to one herpstat. Thats 6 shelves. I have been carefully monitoring temperatures for about a week to make sure there arent any tubs too hot or too cold.

Good luck with your build!