the ET figure is the offset of the alloy, std genuine monza's are ET51, so the ET48 are the ones you want. You would struggle to fit the ET35's.
Genuine 18" Monza's have a corrosion issue, in my experience the reps are worse, and I wouldn't put cheap tyres on such a fine handling car as the mk5. You may be better waiting for a secondhand set of genuine wheels, but 1's in decent condition fetch big money