1. ALWAYS check that the company is registered, even if they have been around for a while, does not mean they are legitimate. (Been there, done that)
You can do this here - https://companies-register.companiesoffice.govt.nz
2. THERE ARE NO FRIENDS IN BUSINESS, you pay them, there is no such thing as 'mates rates' they not only want your money, but NEED it. Been there, done that.
3. NEVER give them an inch of leverage, an inch turns into a mile. If S*** seems to good to be true, IT IS. Check on it regularly, everyday, physically, if possible. Been there, done that.
4. Supply as many parts as you can yourself, never assume they know what you need. Been there, done that.
5. PAPER TRAILS, always save your digital conversations, and if things are bad. Record phone calls. ALWAYS get receipts for work done.
Seriously just never be afriad to be a total C unt. That's your money, if they don't want to do what you say, they'll either say no. Or yes, and take that money of yours.
It's always about the money.