Hi, I need a motor with no torque to speak of (the load will have less mass than the shaft), but which can run continuously at at least 5000 rpm and hopefully as fast as 12000 rpm ('200hz'). The shaft angle needs to be determined by the controlling electronics (a few seconds of 'catching up' would be acceptable). The shaft angle needs to be predictable while the shaft is turning (at a smooth steady constant rpm) and the shaft needs to come to a controllable shaft angle stop. Shaft angle could be sloppy up to 20 degrees or so. So minimal strength and precision and this needs to translate into small and inexpensive. I cannot find any thing that fits the bill in the servo, stepper or synchro categories. Any advice is highly appreciated.
PS I need to be able to vary the rpm from 600 to 12000
Industrial servo motors can certainly achieve 12,000 rpm - but the price will reflect the fact that they are not hobby devices. Bosch, Kollmorgen, Siemens, ABB, Yaskawa, Omron to name just a few, hundreds more out there.
Peak torque and other factors need to be defined to properly size things, saying it is low mass isn't quite enough 
It occurs to me that color wheel motors in DLP projectors probably have the properties I seek. When I can confirm this and when I find sources, I will report back. But in the mean time any other ideas (or suggestions for sources) are appreciated.
You need a small low-spec servo motor, which means a PID loop controlling a DC motor
with an encoder on the shaft for position/speed feedback.
Choice of motor depends on mechanical load and expected lifetime (small brushed motors
can have poor brushes). Encoders can be incremental or absolute, and there are optical
and magnetic encoders available.
The control loop is a classic application of a PID controller, error input determined
from commanded and actual positions, output is drive level/direction to an H-bridge motor
driver.