I'd love for some kids bikes to turn up and be lovingly restored and handed over to kids, so we can get the next generation excited about Viscounts. I mean, we won't be around forever, so we need them to take care of keeping the Viscount pride alive, right? I have already 'educated' a 15-year-old, who got the bike from his grandpa, to look after it and take great care as it's special. Not sure he really understands, but he's riding it almost daily.
As for most common: The Aerospace Sport was normally fitted with TourneyGS, the Pro with Crane, The Sprint with Huret/Sach Huret. I hope that Cusqueno can confirm this as he's got the catalogue (man, I'm still so jealous). I think the high-spec bikes (Aerospace) were all fitted with QR and the low-spec with solid axles.
With regards to getting complete bikes cheaply and using the wheels: can work well, but there are risks that you have to respoke the wheels or the hubs are damaged. Depending on how well they were looked after, or if some cowboy pitted the hubs when trying to remove the bearings. I have that problem with the hubs I built wheels around. It needs some special gap-filler (not sure if we used loctite, definitely not loctite 641) to have them run smoothly. But at least you'd get the rims, definitely great if you get Milremo or Birmalux, but there are very few of them. Weinmann's good, too. If you can get a bike for 'little' money, you also have some spare parts--always a great thing to have.