For a 66 to 68 (nonUS) 105 Duetto, there are some bits that are needed in a great resto that are unobtainable, especially some of the rubber and plastic bits; the car even has some PAPER bits (heater duct hoses)! Some of the repo parts that are available are not quite right, don't quite fit, etc. Finding a car that has "all" its bits won't even do it as these replacable parts can't be replaced.
Then there's the issue that there is a lot of variation in this group of cars FROM THE FACTORY. Zero, one, or two boosters, ATE or Dunlop brakes, red, yellow,and/or white on the turn signal lenses, two or three holes in the hardtop latches........ After all, in this era, cars were built based on what was on hand.
Also, they were not assembled in chassis order. Pinnafarina made the bodies in a variety of colors and numbered them in order, with variations in attachment and mounting brackets over time. But Alfa assembled cars based on the colors ordered or needed, so the body numbers weren't used in sequence. There was also a lot of turmoil in the labor force in Italy during this period, so there were strikes for days, weeks, sometimes months at Alfa or somewhere in the parts supply chain. And Alfa didn't keep the kind of records US makers commonly do (e.g. no build sheets for each car), so it's hard to tell what is truely original.
Since I own (from brand new in 1967!) a Duetto, I've spent years finding and stockpiling some of the needed and hard to find stuff! Still looking for some things.
The good side is that these are wonderful cars to drive. Simpler, much lighter than later versions; they seem to drive and handle more nimbly, even with the "lower" power of the 1600 - or 1300 ('Junior' - Italian only cars) engines. [There was a big tax break on the smaller engines in Italy].
The hardest problem I've had is deciding what kind of car I really want.
Enjoy
Robert