This is a bit of a hack but what I did when I found my 5 speed drill press would not go slow enough for hole saws was to mount a second motor beside the existing motor and outside the belt guard. When I want slow speed I open the top of the belt guard and run a belt from the second motor to the top pully on the motor and use the remaining 4 pulley sizes for the belt to the spindle. The second motor is a half horse off a washing machine so it is two speed as well. A short summary might be that I use the original motor as the jack shaft and pulley. It all works fine and seems to have plenty of torque for running hole saws drilling steel. As I said, it is a bit of a hack but it was free. If I had it to do over, and had the money, I would probably go the 3 phase / inverter route. I have a 1.5 horse on my lathe and love it.
Brian