Invest in a good potting medium (without added fertilizers and water absorbing crystals) and don't forget to add that perlite! Rather than throw it out at the end of the season, just add it to your landscape or flower beds, or even top dress your lawn with it.

Maybe by vision of this is faulty, but it seems to me that a trash can would be too tall causing wasted growing material. You might want to try a rope handled bucket (similar to the muck buckets linked). You can generally find the buckets (not quite as sturdy as the ones in the link, but sufficient) at Walmart, HD or Lowes for about 7 bucks. If I remember correctly, these are about 18"H X 23"D which is roughly 25gallons (or if needed, the volume is just under 4.5 cubic feet)

Also, I am not sure how closely you are planning on following the directions given. They call for a soilless potting mix. You can mix your own fairly inexpensively, or try to find one already made.

~james

Here is a link that might be useful: muck buckets

I always add some sand to my outside potting mix and i can't tell you how much as i go by feel so it has some substance instead of the lightness of potting soil.
vickie