Hi vincelanier, I also try to save money so we can take more trips :) and require our cars, lodging, etc. to be clean, safe, and convenient. We've rented from Apex 5-6 times and the cars were never stinky or dirty. One time, the car they brought to us was decent inside but dusty on the outside and had a spiderweb on one of the side mirrors. We requested that they wash it and waited 15 minutes for that to be done.
We traveled from north to south because I liked the flight schedule better. We flew from the north island to the south island but depending on what you want to see, driving the whole way and taking a ferry may make sense. Apex rental cars are allowed on ferries and Apex will sometimes pay the ferry charges for the car:
When renting in New Zealand, can I take the rental car inter-island
across Cook Strait?
Yes, you can take your rental car across Cook Strait using either the
Interislander or Bluebridge ferry services. If you are planning to
travel between the North Island & South Island, you can book your
ferry tickets at the same time as you book your rental car. Apex New
Zealand Car Rentals may cover the cost of the vehicle ferry ticket
depending on the length of rental and the travel period.
Below are links to resources I used to plan our trips. I hope they will be helpful to you too.
Request a big, free color map of NZ to help you plan your trip
NZ Primer (see comment #4 for a quick intro to NZ)
Tripadvisor NZ Forum (read/search posts for ideas and/or have expert locals help you plan your trip)
Ideas for NZ Itineraries
Earth Trekkers Blog
Dangerous Business Blog
Young Adventuress Blog
AA Traveller Driving Time and Distance Calculator