I purchased a 20' to contain tools + small equipment in move across country, now provides storage.
Only things noticeable or of concern; [a] Condition of door seals, add some sort of awning or entryway [b] Add different primary locking mechanism for day to day convenience, [c] A 20' weighs more than 2 tons. Doesn't need actual foundation, but pads keep it level over time. A precast step or two up would likely assure interior stays dry. [d] Anything a home or building would have is possible, including a regular doorway, windows, skylights, wired for outlets and lights, so the high cube versions add useful ceiling height at minimal expense. [e] Main complication initially will be delivery. A flatbed/ rollback auto tow service brought mine and placed it accurately.