Personally I am convinced I found the best way to play Russia.
In my last game I did not build a single normal infantry (lots and lots of mechanized infantry instead) and I got a mass of troops very fast to could both north, south and east to advances. My luck is that my enemies have not mastered the Italian can opening and feel obliged to keep the baltic fleet alive against USA bonber spam however so this is really in my advantage.
You start with lots of infantry as Russia so logically the first buy is always minimum 6 artillery, you could even opt 9 so the South depending on Germany buy. These artillery greatly enhance your starting troops and later your mechanized infantry. Make sure you lose those initial infantry on both offense and defense while preserving your other precious troops.
Gargantua proposed 6 tanks but I prefer that initial artillery buy and start buying some tanks per turn afterwards. Novgorod and Bryansk are your strong points.
Put effort in holding the Eastern countries and save China as long as possible, this provides Russia with lots of earned IPC while all troops (besides the Siberians) can 100% focus on Germany. For me it it usually Russia north, China west, USA east and UK + Anzac south hurting Japan from 3 sides if it decides to commit to a place. I have absolutely started to love the Guerilla China so extremely difficult to kill completely.
With the S125 bonus and the East I was making around 38 IPC per turn for a long time, buying 5 mech and 3 tanks per turn. Because of all your troops being mechanized (and thus able to move 2 spaces) I find the two fighters and tactical bomber sufficient as airforce and rather buy 2 tanks or a tank + mech instead.