Take it you are going for a diesel heater?
I had the Isothermal 1 from Purefreedom a couple of years ago and found it disappointing.
1. Water does not stay that hot - Isothermal 2 (9kw) would be better, also I use 8mm hose, guess using a 6mm hose would help as water would not cool so much getting to brush.
2. It eats batteries. IMO need 2 110amp batteries solely for the heater. (Purefreedom set it up with 2 85amp batteries which also run the wfp pump).
Not sure how similar their heater is but just forewarning you.
Be interested to know how you get on with the Grippamax system.