I was short of time when I made my earlier post, so here is some detail.
With both DC and 3ph motors you lose torque as the speed is reduced. You can overcome this by using what would normally be considered an oversize motor. I replaced a 1.5 hp motor with a 5hp. I never need 5hp but it gives me acceptable low speed performance without additional gearing.
When most people on this forum talk about DC motors they are referring to treadmill motors. If you are replacing a 2hp AC motor with a DC treadmill motor then if you gear it for a reasonably high maximum spindle speed then you will be very disappointed with the low speed performance. I have done that.
You don't have to spend $1000 for a motor, at least in Europe you can get suitable 3ph motors for < $250