If the machine doesn't have very many miles on it, it doesn't need a new chain unless it's terribly corroded or somehow damaged. If you get a new chain on a low-mileage bike, you will not need to replace the cassette/freewheel as well. If it's something with some miles on it, best bet is just to go for the whole drivetrain at once (chain rings, cassette, chain). I also used to be a bike mechanic. It's crazy to see the electronic shifting on high-end road rigs. Impressive, but crazy.