How quickly can I get a report?
Often a few days, and inspectors are used to auction deadlines. Say the date you need it by in your request, because availability rather than price is usually what decides this one.
Is a report to NZS 4306 different?
It is the New Zealand standard for residential property inspections and sets out what must be covered. Ask whether the report is to that standard, since a cheaper inspection may be a narrower one.
Will it cover the roof and under the house?
It should where access allows, and where it does not the report should say so explicitly. An inspection that quietly omits the subfloor is worth much less than one that tells you it could not get in.
Can I use the report to negotiate?
Frequently, yes. A specific, costed defect list is far stronger at the negotiating table than a general impression, which is another reason to get it before you are committed rather than after.