MT,
Offering to substitute the 12v motor for $35 is a very fair offer.
The only things missing with the heater kit seem to be: the two rubber grommets to protect the heater hose where it passes through the firewall; the gasket/seal for where the heater/blower unit attached to the body, and, the metal ring and rubber seal to surround the heater motor where it protrudes through the firewall. Most of these items are available from restoration parts vendors, except for the metal ring for the motor seal.
I know that you know this, but others might not (you carefully and correctly listed the system for '48-'53): the '54 fresh air (deluxe) heater had a few differences: an inside/outside air door where the blower unit bolts to the body (necessitating a longer control cable and a few clips); and, a slightly different label (at least on the '54 Chevy unit).
Tim
