If I was to get one now I would go for the 9kw. Part of the problem I had with the 5kw from PF is the batteries they supplied were not up to the job, though I still think it was underwhelming anyway.
I would not go near Pure Freedom for a hot system again.
A Thermo Top C diesel heater is rated at 5.2kw. Unfortunately about half of that goes as waste heat out of the exhaust and heat loss through the casing, pipe work and heat exchangers.
I tested a 5.2kw many years ago and was only able to raise the water temp from 9 degrees C to 35 degrees C at a flow rate of 1.5lpm. To raise that temperature of my water took about 2.65kw.
So a Thermo Top C isn't going to do anything more than raise the temperature to make the hose supple.
We have seen a cleaner in the south who has fitted his Thermo Top C to rather heat the water in the vans tank. It will work better than a 2kw electrical immersion heater element but not a good as a 3kw element.
Heating on demand is the most efficient way but a Thermo Top C just isn't man enough. It is brilliant for the job it was designed for - an engine preheater in very cold climates.