Hi Henry
I have used a Rotovac for the last 4 Yrs. Fantastic piece of kit. New would be around £1500. If the Ebay item is only 12 mths old and used twice then it could be a bargain, at the right price!!! If used more then maybe need new carbon brushes. Also keeping the felt rings in good condition helps performance. You will also find you don't need such high psi as when using a scrub wand.
Do not use on low profile carpets, they bounce around too much, your better off just using a scrub wand.
If your using a TM and breaking your back with the wand then, perhaps, you could consider buying a glide. Should dramatically reduce the load.
Adrian