The drivetrain can can be anything from a simple single speed without brakes to a touring bike with a wide range of gears. Most people tend to have too many gears with very small increments between them. Why shift gears if all you need is a one or two tooth up or down, rather just pedal a little harder or faster. We recommend having as few gears as you can get away with for the type of riding you like doing. Having good brakes is essential and your choice of tires and handlebars will usually determine the sort of brakes you can fit on your bike. Wide tires usually require disk brakes and drop bars or flat bars will determine the type of levers you can fit. Shifters are often linked to brake levers, but you can have bar end or down-tube shifters, it’s up to you ( Lance Armstrong liked one down-tube shifter and one shifter on his brake lever).