Many Henry Js were equipped with heaters. Yes, they were add-ons so about half did not have them. There were actually three types built for the Henry J: The first was part number 100238. As pointed out, it is the type used for the 1949/50 Kaisers and it is listed in the 1951 Henry J parts book.
. As can be seen in the photo, the switch mounted on a bracket under the dash which was included as part of the package. That type of switch is KF part number 212957.
Shortly thereafter, a different style, part number 100171 which is the most common one, was introduced and it too is listed in the 1951 Henry J parts book.
On this type the blower switch was mounted on a chrome hanger, usually mounted under the dash to the left of the steering wheel. It is KF part number 212426. My guess is, this is the type you have.
The third type, KF Part number 100302, is listed in the 1952-53 Henry J parts supplement. It looks like this
The switch for that one is listed as KF part number 213773.
And yes, there were a number of after-market units and sometimes people installed heaters from different cars in the Js. I've seen Js with Chevrolet heaters, Southwind gas type heaters and even one with an International Harvestor heater.
I think if you find any 3 position switch, which can handle about 1 amp, it should be fine electrically. If you're looking for an original type, Rudy Phillips lists them in his catalog. His phone number is 1-760-789-3608
Door arm rests were part of Accessory Group 10, 18, 19, 21 and 22 so if the car was equipped with any of those AGs, it would have originally had door armrests. They were also available from dealers so they could be installed in any J so if you're going for a truly authentic restoration, armrests would be considered correct. My '53 J did not have them originally but I added them. If nothing else, they make it easier to pull the door shut when you get in the car. The small handles bend and break when pulling the door shut with them so in my opinion, that's reason enough to add arm rests.