What will a National/NZ First government look like?

Going by the opinion polls, it looks like National will form the next government with some kind of deal with NZ First. It's misleading to call Winston Peters a 'kingmaker,' though. It will work the other way around, as Winston will not have a choice. Key will be 'crowning' Peters.
The nature of the agreement between the two parties will depend upon the numbers of seats that the two parties hold, and on the bargaining power held by Peters. It will also depend on what Peters wants to get out of his negotiations. He often says that policy is what matters to him, but his track-record shows that office-holding counts.
So, the deal could be as loose as a confidence-and-supply agreement with a ministerial portfolio or two for NZ First, through to a formal coalition agreement with shared seats in the Cabinet. The latter could be fraught with tensions, as in 1996–8, and would depend upon a good working relationship between Key and Peters.
If National's numbers in the House turn out to be strong, NZ First could be limited to simply supporting National on confidence and supply (or even just agreeing to abstain on such votes), in return for some policy concessions. But this looks less likely, as Peters will probably be in a strong enough position to negotiate a firmer deal than that.
At his relatively advanced age, Peters must be looking to his legacy and reckoning that this may be his last chance to really shine on the political stage. He will want to be seen to be the responsible senior statesman. I doubt that that he'll play up and cause uncertainty.