8000rpm at 18V is roughly 444rpm/volt, which is slower than a Speed 900 by approximately 100rpm/volt so depending on what voltage you run it on you could go for a ratio around the 2.5:1 mark for a good balance between torque and speed. Although if the motor doesn't pack much of a punch on its own a higher ratio would be the better option.

Any idea of the dimensions of it or the weight?