You're better off getting a quote for supply and install from one place, it's much easier in terms of warranty. Installation requires electrical work, building work, and gas type work, so it's probably cheaper to get a firm to do it than hire a bunch of people.

You'll want them to make sure you get the right size too, and locate it properly. I got a heat pump a few years ago through a good firm, but it turns out it's not big enough - on cold nights it barely stopped running, and heat pumps that are working hard use more power than larger units that are working moderately. I put in a bigger one and moved that one to my kitchen, the new one seems much better.

If you're trying to heat a whole house with a single heat pump then 11kw will be about right, maybe a little small. You might actually need to 6-8kw heat pumps to cover the whole area, or you might look into a ducted system like the Fujitsu sleep pump, which is what I should've done.

I just had a heat pump for an area that I guess is around 60 square meters put in, a moderate sized lounge, wide hallway, and three decent sized bedrooms (that doesn't count the separate kitchen/dining area) I got the largest Fujitsu Nocria unit, 8kw minimum, 11kw peak. It works well, though it's a little loud, the indoor unit has a fan that could just move air around a huge area. On ultra quiet it's still not super quiet, but it's not awful either. My older Daikin is a bit quieter, I think, but pretty similar. Overall I guess I recommend the Nocria, plus Consumer rates them very highly. I paid $3400 which includes some kind of government subsidy, I think it was around $4000 without.