Welcome to the site!

1. PVC is lighter, and overall better than melamine. If you can get ahold of it I would go that route.

2. Are you talking about a recess for the heat tape? If so a recess isn't required.

3. You shouldn't have any issues with those tub sizes.

Helix thermostats use to be the industry leader, but nowadays they are outdated and in serious need of a redesign. The herpstat 1 is currently the industry leader and is the safest thermostat money can buy.