Is the interior of the unit well enough protected to guard against all the junk (concrete blocks, steel fence posts, etc.) that may be loaded and can damage the unit?

Trecan Snowmelters are very well built to handle all manner of debris, and are very easy to clean out. The portable Snowmelter melting tank walls and floor are typically 3/16 to 1/4 inch steel plate stiffened with HSS members. (However, they are not designed to be rammed by front-end loaders or any other heavy equipment).