Out of an abundance of caution, please be sure of whether those decking boards are pressure treated. The newer formulations of copper fungicides are safer than the older methods of pressure treating but are still not exactly great for soil or human health when growing food. In that same vein, thoroughly wash the barrels before you start filling them and be sure to flush any and all plastic bits out after you’re done setting up the outlets.

Your library sounds awesome! If they don’t have one already, consider advocating for a seed library

As for soil/compost/growing medium - bagged stuff is not a worthwhile investment for your money from a cost-per-cubic-yard standpoint, and those products tend to have an inordinate amount of filler that won’t help your plants. You’re likely to spend less and have better results ordering a delivery from a landscaper supply business. They’ll have you order by the cubic yard and dump it at an accessible location at your place. Spread a tarp out to give them something to aim for and to reduce what you lose during the filling of the beds.