The tank will only get heavy when it fails...normally, it holds only a very small amount of water. Precharge it to your normal water pressure, and it will only accept water when it exceeds that amount (like when your water heater runs without a faucet open - which is why you need one in the first place!). The next time you open a faucet, that water will escape, and you'll be back to essentially empty. You might want to check the air pressure in the thing periodically, but they don't usually leak (I've not had a huge amount of experience in this, but mine haven't!).
When (not if) the bladder in the tank fails, it will fill with water as the air is disolved in the water passing by. Then, it will be heavy.
A shutoff to the tank is a good idea so when it comes time to replace it, you don't have to shut off the main supply. A ball valve here is great.
You can install it basically anywhere it is convenient after the pressure reduction valve...doing it on the cold water will probably allow it to last a little longer.
My unprofessional experience.