Hello.
What kind of variance would be acceptable for your application? To have a DC motor run at a steady speed with a varying load will require some kind of feedback. I am not sure what you consider simple or with minimal complexity, but you could use something like our Jrk Motor Controllers with Feedback to do closed-loop speed control with tachometer feedback (e.g. using one channel of a quadrature encoder). I recommend reading through the jrk controller’s user’s guide to get an understanding of how to do this and see if it would work for you. Additionally, my post here (as well as the one I link to from there) might also be helpful for understanding how the feedback works and how to appropriately configure the jrk for your system.
If you can give me more specific requirements, I might be able to suggest a suitable gearmotor. For example, you mentioned a load, but then said it will vary; what is the full range you expect? What RPM do you want the motor to run at (without additional gearing)? Also, do you have a preference for operating voltage?
Brandon