Looks like a worthwhile project!
As for seatpost sizing, I'll leave that to those who know
Ital vegas better. The two '84 Univegas I have are Miyata-built and use 26.8
I, too, love the chrome socks era. My old '75 Fuji S-10S has chrome socks front and rear, as well as a 'claw' rear derailleur. Never been a problem in over 50k miles, other than the pulleys wearing out (and SunTour replacements were impossible to find in the pre-internet days) so I ended up replacing the entire rear derailleur.
IIRC, the brake hanger/quick release is readily available (several on ebay)
https://www.ebay.com/itm/Vintage-Qui...8AAOSwEh1e-jsE
Keep us informed/updated with your progress