With regards to your car:
1. Less than 52,000 Frazers (Manhattan and standard combined) were produced as 1948's.
2. Heater/defroster (labeled as Air Conditioner because it produced "conditioned air") was either factory or dealer installed on almost all Manhattans and standard models.
3. Roughly 50% of 1948 Frazers produced came off the line before the addition of the Darrin Styled badges ended. This is an estimate based on month-by-month production of cars compared to date on the K-F Service Bulletin announcing the end of the feature.
4. If low miles can be documented, this is most unusual and rare indeed Also, having the original sales invoice from the dealer is rarely seen with cars today.
5. Spot lights and other factory accessories (spot light needs the K-F emblem on the top, not the Unity tag or some other branding) can be rarely seen items. It should be also noted that according to Parts & Accessories Bulletins, there are 3 different "legal" types of back-up lights that might be found on a 1948 Kaiser or Frazer (round lens, bezel that looks like the front parking light, mounts on side of bumper guard).
What would make the car truly rare:
A. Leather Upholstery on a Manhattan
B. The Dual-Manifold version of the 226