I suppose you could call me the Pigpen of sours. I'm in the minority with my opinion, but I have made probably a dozen or more sours and I have reused plastic fermentors with no signs of contamination in subsequent batches. Its my understanding that wild yeast and bacteria are susceptible to the same cleaning sanitizing regimens that we normally use. I let my PBW solution sit in the fermentor for a good long while, and use Starsan as well. Its possible that there is an increased risk associated with this practice, but my experience is that it is definitely not a certainty that you'd get an infection from reusing a plastic bucket. I do have a dedicated autosiphon, just because I was following conventional wisdom in the beginning and I already have a couple more now for other beers.
Its not a huge outlay to have a dedicated bucket and racking cane, if it makes you feel better. A lot of the stuff we do by way of cleaning and sanitizing, is more a matter of comfort level than hard fact. Sanitization, by definition, isn't a really absolute deal.