I think there's no point buying a worm farm at all June. between my garden shed and the fence is a metre space. A lot of people have this situation, it's an ideal space for a worm farm. What I did was block off one end with wood and raked the ground into a slope running down from that end. I lined it with Black plastic that runs up the sides of the fence and shed.

I dug a hole at the low end and sunk a plastic half barrel into the hole and ran the end of the plastic into that. I actually leave a 9 litre bucket in there for ease so the barrel only catches overflow. So the space for worms is pretty big :-), about four metres long one wide and 90mm high. i have misters and a shade cloth roof, because even in the sheltered spot it can get hot, I'm in Australia. I use the misters just enough and that space takes all I can throw into it. worms propagate very very very fast indeed.

I fill up one end and then rake it fw'd being careful not to rake the plastic liner obviously. then as worms work their way back into the fresh pile you can remove the cubic metre or so at the front. A cubic metre of worm casting/compost makes a great potting mix when added to some coarse sand and rottted sawdust i can tell you. And the liquid run off just gets chucked around the garden as the whim takes, there's no shortage.