As a Fuji (Nikon) user myself, all I will say is get something between 85 and 110 mm, yes Prime it has to be.
This one was taken with a Tamron 90mm F2.8 Macro lens, costing about $400 I think. When it arrived, I was shocked, I was expecting a nice big lens, I got a little thing, but it works wonders, well worth having.
