The legal side of setting up a business is very simple in NZ and low cost.
If you work as a sole trader you do not have a separate IRD/GST number for your business and you do not need to register.
If you decide to set up a company, the registry costs are from $50 and a quick n easy annual return that is filed online each year. Accountants usually do charge more for completing a companies accounts compared to a sole trader, though you may also be able to reduce your final tax bill to compensate.
Some accountants use a fixed fee and others use a hourly rate which then matches the time taken to your bill rather than a fixed fee. I also know of accountants that use a mixture of the two.
You would (in most likelihood) need to register for GST which is charged at the rate of 12.5% of the sale price.
Good records are essential, though good software is available for a very reasonable price.
Shawn