If you need to buy a pump for around $40 it would make more sense just buying a complete unit on eBay The eBay Link Here

If you have the parts and want to build one for free then Click Here for ideas.

I was thinking if it was not used very often you could use two 20 litre drums and gravity feed the washer from the top drum and then when the lower drum
is full swap them over.

Cheers Max.