I owned two Monzas. And the maintenance matters. The first one was maintained 100% by its previous German owner (I imported the car in 1994) and I never had problems with it (just the well-known rust).
The second one (after number one crashed) was owned by a young man who just had to show-off with the car. Speeding and tanking, no maintenance. The car, though still with the nice 'Monza smell' (you know what I mean) never proved to be 100% faithful to me.
I owned one for five years. I replaced the radiator and had the aforementioned overfueling problem, but just tightened the pipe connections every few months (if they split, I just cut the end off and tightened them again!). The inner wings were repaired and the front outer ones replaced then the car was re-sprayed the original colour. The rear trailing arms had to be replaced just before I sold it (which I regret). I would have forgiven that car anything it was so reliable and so good to drive. If I saw another really good one, I don't think I could resist buying it. But then going back to the pubs of your youth was never a good idea so maybe I wouldn't!
Regarding the fuel problems - most Opel/Vauxhals of this vintage have this problem. The original fuel hose is not up to the job and the very high under-bonnet temperatures do not help. At the first sign of bother (petrol smells, fueling problems) replace the lot. It is very important to use top quality hose and the right clips (the clear rubbish with string inside will not do!!). Expect it to be expensive!
An erratic idle speed is usually down to the additional air valve packing up. A good dose of carb cleaner is worth a try, but they are often shot.
I have four Monza's and they are very reliable mechanically. However if you wish to enjoy the Monza experience better learn to weld.
I do not use them as daily transport but have enjoyed tours around Europe and the UK and have owned Monza's for 15 years now.
There is a company called GM6 in Cornwall who can sutisfy all parts or restoration needs and they are perfectionists.