I’m with Gargantua.
Buy mostly Infantry, with an Artillery here and there.
Take Iraq as soon as possible with a goal of getting a tank down to Ethiopia/Somalia
Take Finland/Norway if the opportunity presents itself; it is worth 11 IPC’s to you and takes away 10 ipcs/turn from the Germans. By no means is this as must do as Iraq, but from time to time it is good.
Germany will either go heavy north or south (if he splits up than he is likely inexperienced and you should crush each army seperately). Retreat from his heavy push and counter as much as possible in the other.
i.e. If Germany goes heavy towards Leningrad, abandon Leningrad the turn before you likely lose it, and slowly move your forces east towards Moscow- don’t give the germans an easy way to kill your russian stack! In the South, hold your ground (and hold onto Ukraine) as best you can without committing more than a couple Inf there/turn (along with your aircraft).
Starting to think the Eastern Front troops are best kept in the East Front- with perhapd 6-8 staying in Siberia and the rest going around mongolia to China. They rarely get to Moscow in time to help anyway, might as well try to protect your eastern lands.