However, if I were to build an IFR
cross-country, single-engine aircraft by starting with a base aircraft and
tossing lots of money at it, an airframe that accepts some type of
anti-icing system would be my starting point.

I was being a bit sarcastic, an Arrow is a good plane. You can always
add full deice to any Mooney J model forward. However, for the system
to be FAA approved for known icing conditions it must have been factory
installed. However, there are a lot of well priced Mooney 231's out
there that have their FAA approved known ice systems still installed
and working.
The FAA known ice also means that aircraft was designed such that
things like fuel vents, etc are vented in such a way that they don't
ice up. Its more than just the deicing system.

-Robert