Don't make yourself nuts with this. Most of the guys in the field cut
to the chase on water heater problems. Just yank the elements (make
sure you replace with the right sized element) and yank the
thermostats and replace it all.
And yes, you should drain the tank regularly. When I drain a tank,
once it quits draining, I like to hit the cold a few times to stir up
any remaining buildup in the tank. You should service the tank at
least yearly. But I know some plumbers who insist on servicing the
tank every three months.
If the unit is older than 9 years old, you should go ahead and replace
the entire unit. I've seen them last a lot longer. But we use 9-years
as a good rule of thumb in the field. I had an old unit in one of my
rental houses that was 40-years old.
good luck with it.