If you choose to use a simple lamp dimmer for speed control, it may have a tendency to wander with varying loads. This is not much of an problem for quick-mixing chems, but for slower, longer-term mixing, wandering speed can become an issue.
Yup. That's the ghetto way out and it might work fine. Certainly worth the $9.99 to give it a try. If it doesn't work on the bench you can go back and reverse engineer the original controller.