If chosen correctly, a French Horn can double as a diapason, and a bright
reed enclosed can replace mixtures. Of course, you can't get all this
pipework from the same era. The horn/diapason should be from the teens and
the bright reed should be from the 30s/40s.
For a residence organ, a good pedal stop would be a Haskell violone 16'--
compact, versatile.
Don't forget that the acoustics of the location should play a very important
role in size and color choices.