I'm trying to find a cheap micromotor or small dremel suitable for drilling, sanding and polishing jewelry and precious metals. I'm not sure if it's possible to find something usable for under/around 100$ but it would be great if someone has recommendations.
"usable" is relative. I bought a perfectly functioning dremel-like tool off of AliExpress for under $30 and it came with bits for the activities you describe above, as well as a 3ft flexible shaft attachment. It took weeks to get to me but works great, however I am constantly aware that if it should break, I'd be on my own. Overseas support lines are beyond my patience threshold.
This is not an endorsement. I'm not even providing a product name or a link. Just offering a source for what you are looking for and testimony about my own personal experience with that tool.