1. "Closed loop" systems just need the same maintenance that a furnace would need. Air filters and occasional cleaning.
"Open loop" systems need to be flushed with a cleaner once a year or so. It's not expensive, but messy.
2. The Heat pump itself lasts decades, and the "closed loop" pipes are estimated to last over 100 years.
"Open loop" systems last about 5 to 10 years depending on water quality
3. In a "closed loop" system the ground heat exchanger is a series of pipes in the wells or pipes in a trench.
"open loop" systems just use one well and draw up ground water.
Either way there is a freon to water heat exchanger in the heat pump and a second freon to air or hot water heat exchanger for the output.
4. in a "closed loop" system there is nothing but pipes in the ground and they hardly ever have problems.
There is a well pump in a "open loop" system.
5. Yes. Where's your wallet?
Heat pumps are great in that you can just keep adding more as you need.
A Heat pumps efficiency is reduced as it's output air temperature is increased. For example you could produce 180 F air or water, but that would be about half the efficiency that you would get at 110F
Heat pump efficiency decreases as you increase the temperature difference between the ground water and the output water. So doubling this temperature difference cuts the efficiency in half. (or more based on the design of the unit)
For comparison: The efficiency of an oil system decreases more slowly as it's output temperature increases since it's based on the ratio of the flame temperature to the output temperature.
0% efficiency is achieved when the output air flow slows to the point where the output temperature equals the flame temperature. Note: most heat exchangers would melt before this point :P