Got an Answer?
Check out the modular houses that are built and fitted out in the factory. They truck them to your property and use a crane to put them down and fit them together. Instant house! (Besides the waiting for them to build it).
Have you looked at using shipping containers, and having them fitted out by a builder. There are a few of them on the website "living big in a tiny house".
I understand you can get them imported from China, all fitted out inside, although personally I would be suspicious of the quality, after the problems of poor quality Chinese imported highly flammable cladding, and poor quality steel that fails.
Containers are very robust and well built, so you get a waterproof exterior, but they have to be well insulated. Its worth investigating though.
Have you looked at those home kits where you choose a design and its just needs assembling?