The answer to all your questions are on the interwebs, like this one: https://www.antiquetrader.com/collec...ntage-bicycles

Just like with vintage cars and motorcycles, you want to find, limited production, hand-built and most likely European. The more exotic the better. Mass production bikes will never command money. It’s tough to predict which will or will not appreciate significantly at this point since most collectors already know the market and have already purchased the highly desirable bikes.