Good, straight forward questions!. A closed system is created when there is a check valve between the water heater and the city water main. This is usually in the form of a bell-shaped pressure regulator which is normal located very close to the water pipe's entry to the house. Some new water meters also have a check valve. When water heats, it expands. Since water does not compress, this expansion has to either be absorbed somewhere or the pressure within the closed system will rise to the point that the T/P valve on the top or side of the water heater will trip. While the leaking water is a PITA, this prevents the heater from exploding which could ruin your whole week! A thermal expansion tank is a canister with a rubber bladder that is charged with air pressure to approximately the same as the incoming water pressure as regulated by the pressure regulator valve. It is located in the cold water line somewhere between the PRV and the water heater and acts as temporary for the expanded water. The pressure gauge you refer to is an inexpensive gauge that you can purchase at any hardware or plumbing store. They can be attached to any fixture with a hose thread. You must have one to set a PRV and balance the pressure in the expansion tank. Expansion tank installation is not difficult, but like anything, it does require some basic skill. As noted, the location is in the cold water intake line. You cut it open, install a tee to which you would have already install the adapter to attach the tank to. The tank can be mounted in any position desired, but it has some weight, especially if/when the bladder fails and the tank fills with water, so it must be supported well. The carton it comes in has illustrations of possible ways this can be done. So, about the only skill you need is to be able to sweat copper. Air pressure is adjusted with an air pump through a Schrader valve, just like a tire. They recommend using a bicycle pump rather than a big compressor as the actual volume of air is quite small, but with care and common sense, a regular compressor will work.