Help with a very complex aquarium setup?

I am currently designing an aquarium that is built into an entertainment center. I was just wondering if there was anything that I haven’t taken into account that would make this difficult.

My design requires the tank to be somewhat inaccessible, but you can still get fish and rocks and stuff in and out.

Now the few specific questions I have:

Is it necessary to change the water? If so, my idea involves two valves and outlets in the exterior that can redirect the flow of the pump to a bucket outside, and then suck new water from aforementioned bucket and put it back in the tank. I can’t find the valve I need for this! Any idea where I could find it?

Is it ever needed to turn off the heater in the tank? Just wondering if I need a switch for that.

How easy is it to build a fish tank? I was just going to buy some plexiglass.

It’s a 40 gallon tank.

Thanks for all your help!