Hi, no matter going by air or train, you should make a transfer at Hangzhou and then change to plane or train to Fuzhou.
By train: Suggest you take K163 from Hangzhou East Railway Station at 15:13pm to Fuzhou at 05:21am. It is faster and would take you 14 hours. The ticket fare is about 300 yuan for soft sleeper.
By Air: THe ticket is not much expensive then the train ticket, for one hour.
Hope it helps!