You can always get/make a divider and use it for the T3. You could also get the T1 now, and when it comes time to switch him, buy the T3. You will have the empty T1 to put something in!

With the indirect belly heat (at least with the racks), they use heat cable and slide it into a groove. So the actual heat cable does not touch the bottom of the cage/rack.

I bought the rack with the indirect belly heat, and I hated it. It ended up frying my helix thermostat. I replaced the heat cable with flexwatt.