I can clear up some of the upgrades:
Mortar:Go from a smaller length of mortar barrel to a 120mm, producing that largely feared "Oh Shit Arty" feeling in early-mid game. Well worth it.
ZIS-2: I don't normally buy the upgrade because I use Tank Hunters, but if I do, the ZIS-2 gets a large penetration bonus, and I find it becomes very powerful.