Not sure how serious and efficient you want to see the country. I have limited holidays, so I specialized on effective travelling.
I only went there for a long weekend to see Oslo and Bergen and a Fjord on the way that belongs to the WH list. It cost me a lot. Car rental was reasonably prized. Petrol, hotels and even the food in super markets were incredibly high.
I guess an effective way would be to buy a
fly pass from Wideroe. Grants you 2 weeks unlimited flying within the country for 2 weeks. They fly to Hammerfest (close to Alta), Roros, Sandnessjoen (close to the Vega Archipelago, Bergen, Bodo (close to the Lapponean Area FIN WHS) and some places near the big Fjords. For the Urnes Stave Church I'd recommend a car. I didn't have time to add the church on my return car trip from Oslo to Bergen, but I will do it by car in future.
I went in February and streets were in good shape. But I know from a friend that they cancel a lot of flights during winter due to snow storms and most important, days are simply too short and nights too long.
If you go for the train, keep in mind that you will spend a lot of time in trains to see all the WHS. Eg Oslo-Bergen over 7h, Oslo-Roros over 5h. The long travel times also mean that you will have to spend more nights in the country which means more expensive hotels and more expensive food to be paid.