Your flow temperature will depend upon your emitter design, your air temperature will depend upon your location, your sizing will depend upon your load.
So to answer your question
1) You need to do a full room by room heat loss calculation to BS EN 12831
2) You need to design your emitters to cope with your desired flow temperature (the lower the more efficient the heat pump)
3) you need to know your location so that you know the air temperature to design to
4) you need to know the CoP's of the different heat pumps.
5) don't believe the labels on the tin
a 15kW heat pump is more usually a 10kW Heat pump at 0/35
Hence a simple design will take a professional around a day to do properly
You can find out how to do it here:
Microgeneration Certification Scheme - Installer Standards if you follow all the guidance there it will take you about 120 hours to work through it (just for heat pumps) you'll then have half an idea of how to get it very wrong
like about 95% of the installations done pre february 2012, and about 75% of the ones done since that haven't been done by MCS certified installers.