ya a treadmill motor is gonna be your cheapest bet. plus i think you can use the stock controller for the treadmill. I think it just takes a potentiometer input. only problem is having a system running 120 volts DC which could do some serious damage if you short something out or get shocked
threadmills motor are usually running on 90vdc... keep in mind that a golf cart motor rotor's is only supported at one end you'll have to fabricate a custom bearing mount/motor holder. here ya go https://www.surpluscenter.com/item.asp?UID=2009050217043743&item=10-1899-B&catname=electric