Hopefully the dead one will just be a cable or something simple!
If you do get one where the brushroll is working, then spend the time stripping the brushroll down and cleaning the pcb and motor, I guarantee it will be covered in crap and that wont be helping it keep cool! Can of brake cleaner from halfords or similar will do it.
I wouldnt trust even a working one to pay much over £20 for it, unless you planned to keep it (which knowing you you wil) so maybe £30-£35 and put the cost of a brushroll motor and pcb to one side!
The valve that sorts the hose out with suction may not be moving about properly, or as mvacs said just a nice simple blockage!