Be 110% sure you do the heat requirement calcs right, either over or under sized will be inefficient or ineffective.
Make sure it's sited and installed where noise won't be a nuisance...
Consider a decent size buffer tank / accumulator in the system as they're most efficient if not doing tons of start / stop cycling.
Consider whether the customer wants to be able to take advantage of cheap rate tariff electric (economy 7 type of thing), a well designed system should be able to put this in scope.
Check the size of the heatpump and it's current draw against the mains supply capabilities, big HPs may be reaching into the realms of 3 phase power. Especially if they are on the borderline of acceptable on a single phase supply, as the inrush current at startup may create issues with other circuits further down the line.
Try and build in some contingency for back up heat to water in case of failure and as a boost.
Oh, and check that it's covered in their planning consent... There can be times when additional approval is needed.
One or two things to think about in advance but potentially very good if done well
Good luck!
