I think you may have to ignore the stated amperage as you suggested. 3 amps is 72 Watts at 24v but that wire is built for more. If you go big on the power supply at least you won't have to buy twice.
It seems to be a very power hungry motor. My 24v Parvalux motor is rated at 8 amps and the wires are painfully thin. Despite this you cant stop it turning. Loads of torque.
I would definitely ignore the "3.0A Max" rating, which only give this 24 volt motor a power of 72 Watts. That would be nowhere near enough to move a wheelchair. The "450Watt" rating equates to 18.75 Amps at 24v, and that current draw would match the gauge of wire used in the motor leads. I would guess that moving a heavily-loaded wheelchair at 4mph up a hill, this motor could easily pull 18 Amps from the 2 x 12v 12Ah ( or more ) batteries.
However, in a bead roller I would not expect it to draw that much - maybe 10 to 12 Amps. When you mentioned ordering a power supply, that implies you will be running it from the mains. You could start with 2 x 12v batteries in series and check the current draw under load. ( However, most basic multimeters will only measure DC current up to 10A.)
This eBay supplier does have a 480W supply at £30.89:
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However, the 15A at £23.99 is probably adequate.
This 20A speed controller, at £8.19, comes with a reversing switch:
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