I would advise choosing a small ship, as larger ones cannot negotiate the naroow channels between islands that are the best part of a fjord cruise. What do you want to see? Olden has trips to the Brikstall glacier, Geranger has the most spectacular scenery, the Lofoten Islands are very pretty, the North Cape is a wonderful experience, if the weather is kind and not foggy, but Svalbard (Spitzbergen)is best for wildlife.
Probably best to choose an inside cabin as you will spend very little time in it (24 hour daylight, so 24 hours scenery). For the same reason the onboard entertainment is not very important either. It can be cold north of the arctic circle, even in July, so a few warm clothes are needed.
Norway has the reputation of being expensive. It's no more so than London, but be aware that alcohol prices are very high so if you want a beer ashore be prepared to pay for it.
You'll have a fantastic holiday which ever itinery you choose, so go for it!