It is probably cheaper to buy a used vintage multi-speed bike that to upgrade your single speed bike. Sturmey-Archer makes a 3 speed hub (S30) for a 120mm spacing. Sturmey Archer When you add the price of the hub ($75) plus the shift mechanism (Sturmey Archer) and cables ($25) to the cost of building a wheel - spokes, rim, and labor, you have a pretty costly upgrade. At that, you still have only 3 speeds.