As i mentioned, the car alternator idea is simple because of the following characteristics:
1) it revs to around 18krpm (easily fast enough)
2) it is easy to couple too, either directly with a bellows coupling, or via a polyV belt, meaning you can adjust the speed / torqur relationship if required
3) Load control is done simply by the rotor excitation, meaning you can control the brake torque with just a 5S bench top CC supply (get one with RS232 or GPIB etc and you can automate everything)
4) Car alternators are tough! They run at up to 160 degC in the native application, have no permanent magnets to demag, and even come with a built in fan to help cool themsleves
5) the input shaft has very good bearings, because those bearings normally take the FEAD belt loads, so they are super robust to any input shaft loading (from out of balance DUTs etc)
6) CHEAP! Ebay, gets you one for literally a few £, if you somehow break it, then just buy another
7) you could do with a load at all, just short the phase windings together, all current just gets turned to heat in the windings themselves. This would need a seperately driven cooling fan to extended operation possibly, but the high thermal inertia and high operating temp mean it would almost certainly run for 5 to 10min un cooled (especially at reasonable rpm where the built in fan starts to work)
The overall set up is both simple, and can be quickly "trialed" not the case with a system that needs complex power convertors etc
Those factors make it a winner for me by a fair margin!