Thanks to everyone. Great brains trust here and lots of experience. The overall understanding I've received is that container homes are not a great idea unless you have the right set of knowledge, skills and equipment (or people who can do the work for you professionally). And, the resulting space may not be great plus there can be additional structural problems down the track. So thank you again for all your comments. I've learnt a lot and the decision is clear for me now. A container home is not the ideal 'granny flat' for me.