If I was doing this I'd start with a different motor. The wiper motor from a car. It rotates at a lower speed and is more controllable. The crank arm that is attached to the motors shaft is ready made to mount the magnet. Even though a wiper motor is a higher amperage motor, that is due to the strain from the wiper linkage, running as a stirrer there would be next to no amp draw after it is turning. A wiper motor and battery charger and you pretty much there, need more than one stirrer? Add another wiper motor. With a little ingenuity you could even make one motor do all four stirrers. (Think radial airplane engine)

Running a computer fan at a much lower voltage, won't it cause it to heat up?

They do cost considerably more than computer fan motors. I have about 7 of them here
so I can afford to blow them occasionally. Ideally, I'd want the ability to spin the flea up
to at least 1000 RPM. I'd want a hot plate to heat the beaker's but the most exciting thing
(for me) would be the inclusion of a ultrasonic transducer to the hot plate. This is so I can
ultrasonically disperse particles.

It's all great stuff, only I'm liable to kill myself trying that kinda thing, but that is a great idea
and I'd totally want one of those, still not paying £700 for it mind.

 

And here (US), they can be obtained at a junkyard for under £4