Yes a 34 engine and box will go into an r32.
I would say if you can afford a r34 gtr and like everything about the way its looks then you should get one. They are the newest so are less likely to be thrashed as hard and as long as say an older r32 though they are still driven hard by their owners like all gtr's.
They have the steel wheel turbo's on them standard as opposed to the r32 and r33 with ceramic and the r34 gtr head is a little different with different cams, from what i have been told by mechanics, so upping the boost on an r34 with a tune should see some good results without changing anything. All round they are a better built car with nissan making some good changes, as you would expect from over 10 years of advances from the r32 to r34.