You might be a little careful collecting rain water. Algae will begin to build up inside there after using it a few years, which is not always a terrible thing, but it does get kind of slimy & can be persistent kind of like mold, it wants to come back.
When my plastic drum is full of rain water, I cap it off to keep the mosquitoes from living in it, also seems to keep the rainwater fresh...longer when capped off. And you might consider making a filter (flannel shirt) for the rain water, there is a lot of pollen (which kind of gives the algae a head start) and if you have a maple or oak tree nearby, those can cast off a lot of debris during the spring rains.
If you use a inlet filter of some type, you might make the filter quickly serviceable during those heavy spring rains, because various debris will accumulate quickly. If you have fire place soot that accumulates on your roof, if it becomes concentrated in the rain barrel that stuff doesn't help plants grow much.
Hard Freezing could present a problem, but those drums are very tough.
At $3 each you should be able to triple your money, just by bringing them home. Once you have rain in a barrel, you might be surprized how quick 550 gal. can be used especially while working in a new garden place. Mulch & compost not only conserves water, but it improves the dirt.
Charcoal in the dirt is not a bad idea, unless it is fresh...then it still has some caustic properties to it, takes about a 6 mo. for it to mellow out while in the dirt, some people soak their charcoal in various concoctions. If you can pulverize your charcoal to dust, it blends easily with the dirt, that way you won't be running into the same old pieces of charcoal year after year, the chunky charcoal kind of acts like rocks when raking a row & might end up at the end of the row along with rocks, roots & other debris. Charcoal chunks if you have a lot of it make a 'different looking' top dressing type mulch that is reuseable, like around a shrub or tree, won't prevents grass from growing thru it like leaf mulch does.