Yep, Givi rack is the best one to go for. Model you need is the 340F, which covers both square eye and fox eye models, boxes comes with slightly different bolts for each. The plate you want is the M3 monokey one. This includes the silvers bits that go on the ends of the rack arms too.
Hunt around second hand, they come up now and again, but do make sure it includes all the fitting bolts you need for your model of bike. M3 plates are more available on ebay, as these are common to many bikes. Only the 340F arms are unique to the FZS600.
Any old Monokey top box should then bolt on. Once you've got the rack fitted, just hunt around local and see what comes up cheap. I managed to pick up an old 52ltr Maxia one for £35.
I did a bit of a write up on my blog on the fitting of these racks, check it here:
Givi Rack & Top Box on Fazer 600.In fact, I've just had to replace this, as I bent the hell out of it and snapped the plate in a recent off.
Just came to £100 delivered from Motorcycle Planet (arms and plate). The first one came direct from an Italian ebay seller, cost me ~£65 delivered, but the strong Euro makes this less favourable now.