Book stores and bike shops have lots of good books that provide an overview of bikes and bike design. The questions you have asked get hundreds of pages of discussion in those books.
A "good" used road bike from the 1985 to 1995 era often can be found for around $200 to $250 at a bike shop, or for as little as $25 at Goodwill or Salvation Army. The best of that bunch weigh between 21 pounds and 23 pounds, have excellent wheels and excellent drivetrains. They perform about as well as a typical 2006 road bike selling for $500 to $700.
What is the difference between a 2006 road bike selling for $600 and one that sells for $1,200? Very little that makes a significant difference in the quality of performance on the road. A $600 road bike provides about 90% of the performance of a $1,200 bike. And, a $1,200 bike provides about 90% of the performance of a bike selling for $2,000 or $3,000.
Any road bike that has first quality wheels and tires and that fits you well and weighs less than 23 or 24 pounds can provide excellent performance for fitness riding, recreational riding, and commuting. If and when you get a racing license, you could benefit from a 16 pound bike, selling for $3,000. But, for most cyclists, a $3,000 road bike is as "necessary" as having a Indy racing car for driving to Krogers for a loaf of bread.