1. For the thermal boiler thing, I would use the energy input hatch. I have no idea where you would put it, but that's the block that makes the most sense.
2. Hydroelectric wheels would work by either pumping in a ton of water, or require a waterfall. However, without a form of finite liquid, they would be extremely OP. They would have some machine casing, a main block, a dynamo hatch, a turbine/waterwheel slot, a maintenance hatch, and maybe an input hatch.
3. MSR. Molten Salt Reactor. I personally hate lava, so this is probably the fuel I would use in the aforementioned thermal boiler.