Keeping out pests...ideally raccoons, toddlers (though since my intention is to protect them it's debatable whether they count as 'pests'), and predatory insects. If I opt not to keep fish in it (which I am leaning towards in view of the apparent difficulties of keeping predatory insects out of these things) then only raccoons and toddlers will be of concern. I do see deer out back on a semi regular basis, but never outside of the cover of the forest...they are probably not brazen enough to come over and try to get at the plants.
True. I did find some home depot storage bins that claimed they could hold several hundred pounds on the inside, but even assuming that was accurate they would still require not filling them all the way to the top with water (which wouldn't be an awful problem, in all fairness...). Since the storage bins come with lids, I could simply cut two holes in the lid (with a brace in the middle of the lid), cover the holes with hardware cloth (held in place with cable ties), and keep the lid attached if not doing pruning or water changes.
EDIT; Here is the actual pond I am most favoring;
https://www.tractorsupply.com/tsc/product/tuff-stuff-products-tuff-round-tub-25-gal?cm_vc=-10005#