How do metal cases work with connectors & isolation?
The simple answer is to use a plastic enclosure.  The next simplest answer is to buy parts that don't make electrical connections to the case.  You may have to look at data sheets to figure this out.  With some connectors, like BNC connectors, there may be a description of "Isolated" in the part name somewhere.

Other approaches would really require you to look at the data sheet for each part as you buy it, know what electrical connections get made to the case, and be ready to handle them.  For some parts, insulating hardware for case mounts is available.  For others, no such luck, and you have to jury rig your own.  If every connection to the case is Ground, for example, you should be OK (but there may be electrical safety concerns).

For best safety, Plastic enclosures and no metal parts that can be touched by a user is the way to go (double insulation).