I've seen "carbon steel", "heat treated carbon steel" (is that same as
HSS?), and "alloy steel" tap & die sets. For relatively low-use, does it
matter which type I buy? Any suggested sets/sources?

The material I will work on is cast iron (as in TS top), sheet steel (as in
the legs of the TS), and aluminum bar. The most critical use will be to
cut threads in sheet steel for adjustable feet on heavy tool tables.

Along same lines, is titanium worth 20% extra or so on fortsner bits?

TIA. -- Igor