It is easy to get around in New Zealand with speaking English. People are speaking a cute British sounding dialect. It is easy only to fly to New Zealand, have a car booked and nothing else. Tours can be booked in iSite Tourist Center in nearly every city. Campsites need no booking, only go there, have a look and decide if you want to stay.
Do not underestimate the sun, it is really easy to get a sun burn. Always take sun cream to be prepared. To be protected against the local mosquito it is better to buy a local product. The bites from the sand flies became huge.
New Zealand has a good infrastructure and signs can be found for every bigger tourist attraction. If you are going for the hidden gems, roads get smaller and graveled. In case you do not want to rent a car or a camper van, there are many overland buses available. Be aware that the average driving speed is 70 km/h, mostly it will be even less.
I can only speak for camp sites, as we only stayed there. It is not necessary to book anything forehand. Just go there and check out if you like the place. The smaller camps need to be paid in cash. We had a camping app on the phone which worked offline, it included a good numbers of camping sites with description of size and equipment. From small, simple to big, full equipped campsite everything can be found. We liked the most the campsites of the Department of Conservation, which were mostly at remote locations with simple equipment.
In general all of the places take credit card. Smaller camp sides only accept cash.
New Zealand is an outdoor paradise. Hiking there is number one activity. If you are going in the off-season to NZ (October/November) it is not necessary to book the Great Walks in advanced. We just went to the iSites and booked it. Doing this in the high season could leave you without a booking. Beneath the Great Walks there an endless possibility for hiking.
This really dependent what you are aiming for, but if you plan to go to both islands take 4 weeks. Do not try to cover both islands in 2 weeks, this will keep you mostly on the road. Traveling from the South to the North island with the ferry will take 1 day. Also do not underestimate the time shift which is 12 hours + for Europe.