I would definitely recommend Apex - we hired two cars in NZ earlier this year:
One for the north, one for the south.

We used http://www.vroomvroomvroom.com/ and hired through Thrifty. They upgraded our car when we arrived due to double-booking but this worked in our favour.
This site gave us a cheaper price than Apex for the time we required.

Apex however came through in the south island -
We picked up a car on a Sunday in Picton and found all the larger commercial either had a Sunday rate, opened late, or generally were quite expensive for the days we required.

Apex saved us hundreds! No kidding!

Apex is fine. You might also check Avis and see how they compare with a coupon discount.

Tip: With Avis, you can often get a car for NZD$19 with a tank of gas for 24 hours if you are willing to relocate from City to Airport. I've done this in Auckland and Christchurch. It works out well for seeing some of the outlying sites such as Auckland's Museum and some of the volcano viewpoints on the way into town. Best to arrive in Auckland just past 6pm, when you can park for free at any "P" street space until 8am the following morning.

http://www.avis.co.nz/car-rental/con...50&IID=air1way

where could i get the coupons?

The Milford Road is completely safe, it's the gawking tourists that aren't.

You can find some coupon codes in travel magazines or websites, or if you have an airline, Costco, credit card, Autoclub, etc, try calling and asking what the discounts are.

avis.co.nz has a lot of specials listed directly on their websie.