I personally like to keep with the idea of allowing each nation to have the possibility to access each of the new units, because Even Japan and Italy had some heavier tank designs and prototypes, but they just never had the production to actually put them into full production. With the research system proposed however it does accomplish your goal to an extent. Since each research would be 4 stages to progress through, if a country has no bonus to what stage they start on it can take quite a few turns and money to get some researches especially if one has to roll a 6 or a 5+ with one die to progress one stage. Italy does not have the money to spare, and Japan probably wants to spend more money on ships and taking islands and tank upgrades wont suit them unless they are trying to crush Russia. I could also add another research of mass production which the US, and probably Russia has a bonus in, and it will reduce the cost of all of their units by one IPC.
You can find some good light tank pieces on Historical Board Gaming, they are about 0.65 cents per piece
Thanks for the inputs I think I will go with the advanced mediums, and I am currently taking a look at the board to see how some of these could be included in the starting setup
