Yes. As long as the heat pump has been installed correctly and sized up properly for the home. This doesn't mean that no other additional source of heating is required however, the system will require an immersion heater to be built in to the hot water cylinder as a source of heating to get the water to above 60 degrees once a week to stop any chance of legionaires disease breeding if the unit system is providing domestic hot water as well as central heating.
Strictly speaking yes but you can get away without one. If the system is going to be used for domestic hot water as well as heating, then definatly yes so it can incorporate the immersion heater and also as reserve, if the temperature is below freezing outside then the outdoor unit will start to freeze up due to the moisture in the air. Once the outdoor unit has frozen up it will need to switch into 'defrost mode' in defrost mode it will stop providing heating for the house as hot refrigerant is used to melt the ice on the outdoor unit. A buffer tank will hold hot water for the times when the heat pump cannot heat the water up, stopping you from ever being stuck without any hot water.
The only time you can really get away with this is if you are using a smaller system which is mounted in the loft, this will greatly reduce the time the unit spends on defrost mode.
Possibly, the general rule is that the radiator should be 20% larger than you would normally put into a room of the same size, this is because the water coming out of the heat pump will only be max 55 degrees rather than the 70 degrees that will be coming out of a standard boiler. The lower the temperature the heat pump is running then the more efficient it will be, so the larger the radiator, the lower the heat can be set to, saving you more and more money.
Air to water heat pumps are designed to work best with underfloor heating systems, because of the lower temperature that is required by an underfloor heating system, the air to water heat pumps are ideal, because the heat can be set as low as 25 degrees, there is no need to connect the system up to a mixing valve to mix cold water into the system, cooling down the water that you have just spent energy on heating it up.
Very little maintenance is required to an air source heat pump, once a year the outdoor unit should be checked and cleaned of any leaves or debris that may have accumulated in the system, but that is all the maintenance that will really be required to a heat pump system.