2. Always allow more money
3. No build is perfect, but keep a hands on overview of things and you can pick up any issues before they get to far along and become majors.
4. Contract is the law...triple check it! When the dust settles the contract is all you have.
5. You can never have too many power points or cables, and there is no cheaper and easier time to install these
No personal experience but I know of 2 people that have been burned badly by GJ Gardener home builds. Their master franchise also ends up in the news or Fair Go from time to time (personal observation). Knowing what my friends have been through I wouldn't touch them personally.
Word of mouth is the best way to find someone, and ask to see some projects in progress or finished. If possible talk to the owners.
That's my 2 cents worth from 5 house builds so far.