I can't find any reviews either.......... except for sales guff.
It has been given a few awards though (bottom of page)...
https://www.pichlerluft.at/heat-pump-combination-unit.html
I've no reservations over the product, only its suitability for your property. Before signing anything, I'd be looking for a full assessment of your building layout; insulation levels; level of air-tightness; heat losses etc.
This is a simple survey and checklist.
I'm not talking about salesman's burble, but an assessment by Pichler's Technical people as to which product is suitable (if it is indeed suitable.)
One possible worry is the stated 1.3kw space heating capacity.
I would like to know if that's enough for your situation.
In the old days, you would just bung in a boiler and crank it up to suit. It's not like that with this kind of tech..
Forget salesmen. I'd want to speak to Pichler Tech. team direct.
Your extension can be easily built to a Spec. suitable for this product.
I'd want to be convinced of suitability for the rest of the house.