The 3.4 DOHC is a very good motor, the problem is anything around the engine is expensive to have fixed. The motor is a non interference motor.
I drove mine till the belt broke at 130,000 miles original.
alternator $400
timing belt $400
starter $275
intake gasket 325 -$350
spark plug wires $150 a set w/ tuneup $425
The engines run forever its just everything else
This is what I have experienced
BTW, when the 91' Z34 first came out, they did have a rash of engine failures, but this problem was fixed with the 92' Z34
Mine has 140,000 still runs like a scalded cat or it will after I get the leaking intake fixed -
Mark