With my plumbing experience [although not involved the hot water field] I would not recommend a heat pump unit,they can be noisy,expensive to repair ,as they have an attached motor that needs to be serviced by a fridgy/aircond trade.The energy savings only were only approx 30% over electric heaters depending on your tarrifs on the old and wired method on the heat pump.They became popular a couple of years ago when our cash was being splashed around by various govts,and every person who wasn't installing pink batts where installing these heaters. The dooms dayers where saying that storage heaters would be phased out and you needed to look at your alternatives. Then in recent times govts have realised that this would have been impossible to implement and create unrealistic practicable and financial burdens on some house holds .If you are just replacing a off peak storage unit then it is still a good idea to consider a straight swap with a similar heater,as the exchange price is your cheapest option,the running cost are still ok at approx 10-15% of my power bill ,and only have 2 things to go wrong in heating water 1=Element,2=thermostat and both cost less than $70 . If your idea was to save energy,cost was not an issue ,an you have an ideal north facing roof elevation that is structually sound to take a 300kg weight than a closecoupled solar system is a good way to go, if the roof won't take a heavy load than a split system is ok but then these rely on a circulation pump . All the solar systems rely on an alternative heat source for backup, and the units with an gas instant unit attach are the pic ,but you would then require a LPGas connection that would add additional cost.As for brands then i would suggest that you stick with the larger brands, Rheem, Dux Rinnai, Edwards etc. Hope this helps.