If I understand the question correctly... the answer is NO. Both of the external valves (the pressure relief valve, at the top and the drain valve, at the bottom) can be replaced. Usually these...
I assume you mean a relief valve on a hot water tank. According to the tag on it it should be "popped" once a year by the home owner or a professional (Be careful not to scald yourself if you choose...
Human nature being what it is, wait until the first sign of water that shows up in the catch pan that the heater sits on. This is definitely time to replace the tank.
Thermostat replacement.
Yes it can be replaced, if it is of the replaceable type. If you are in doubt about how to do it, perhaps you should have it done by a plumbing professional.
Probably more, but one way to tell, is if it's leaking and it's old
Human nature being what it is, wait until the first sign of water that shows up in the catch pan that the heater sits on. This is...