You should be able to expect the weight of your assembled retro bike
You should be able to expect the weight of your assembled retro bike to be near the 21lbs mark if you build it up with an eye on the scales. Remember though, carbon forks are out so if you want to save weight you'll be looking at lighter handlebars, a lighter bottom bracket and a lighter saddle from the same period, even if it does take you a while to get comfortable on it!
In a word: yes. Leather for bicycle saddles is similar to natural down for jackets and sleeping bags, in that no better performing synthetic material has ever been developed. Leather is cool and non-sticky on hot days, and provides comfort even on extended rides. Most importantly, a leather bicycle saddle will never leave you with that ominous numbness between the legs caused by pressure on the nerves and blood vessels in a delicate area of the body called the perineum.
