They each give two free advances when built, to help you keep up with the Japanese. The Medical Corps HQ also gives you 4 happiness in each city. You will have to alternate building military with building improvements to keep the citizens happy. Also, keep an eye on your happiness slider. The various improvements also increase your city production by a lot.
You should be able to upgrade the M2A4 to M3 Stuarts, but that is something that I have not experimented with as yet. There is not building that really functions as a barracks in the scenario. I will try to figure out what is going on, but you might have to build the M3 without being able to upgrade the M2.
The British in India do not have Matildas, while the Aussies and New Zealand have them. If you want the British in India to have them, use the editor and locate the Australian Matilda, and add British India to the countries that can build.
You might be able to upgrade ground units at the General HQ India building. You should be able to build multiple ones of those, and use them like a more conventional barracks. The was a best a beta release, and I am still working to figure out all of the bugs. There are a lot of them. For one, El Justo had all of the standard infantry units listed as Wheeled, so they could not enter Jungle or Mountain terrain. There is a lot of both on the map. The only foot infantry units were the specialized ones that were autoproduced, and the US Marines. I switched all of the Allied infantry to foot units, but being nasty, left the Japanese as Wheeled. They will be hard-pressed to invade India from Burma.
I taught a class on World War 2 for 13 years, and developed a strong dislike for the behavior of the Japanese during World War 2. That extends to the game.
I may be away from my game computer for a week, but I will keep checking here to answer questions.