Still not sure I understand why you pointed out to my why it was in a shed. Regardless I do agree, a barrel can be used for a filter. That said I personally find them harder to plumb into the system. If you want it under your tank you need a really tall stand unlike if you have a regular sump they usually fit no problem.
That and K1 is really expensive, it would be close to $400 to fill a 55 barrel from the prices I've seen. It is an option but I feel like it can be done cheaper and work just as well.