We should start with the principle that in OOB play, or even modest bid play, Russia has so many structural weaknesses that no matter what you buy, the Axis should be able to detonate you by turn 6-8. Exactly what type of game you are playing will color the answer to your query.
In OOB play with bid, the bid often goes to making the Allies tougher, outside of Russia. In this case, you are likely to get Stratbombed into the history books, which means that you will have to frontload your bulk purchases of 3 and 4 cost units early in the game when you still have 19 production slots. Within a few turns, you are likely to have base 13 production, with some damage–so that buying more expensive units is better later, because you need to heal fewer slots to produce them with. Leaving factories damaged is a key part of cagey play but you will have to fix something sometime, or produce nothing.
In BM air interception (whether you play with the full suite of BM or other mods), you are less vulnerable to SBing. Still, I suggest bidding at least 1 fighter, and buying at least 1 more on turn 1. With 4 fighters, you would be able to deter most unescorted bombers, and retain your production slots for later. SB is the easiest way the Axis can paralyze you, and they can still do it with BM rules, since you as the defender need fighters to take advantage of the rule. Plus, you cant cover more than 1 or 2 factories at a time.
Many players offer up a stronger offense–buying tanks, or artillery. This idea is pretty good for forcing the “E Poland Stack” where Germany can’t stack up right next to you for fear of getting hit before he can present a solid positional defense. Unfortunately, attacking with Russia only hastens its demise, and so this is largely relegated to threat rather than being able to survive attritional warfare. There are many games where I buy artillery but after 1-2 rounds of offensive or defensive battle, have few or no infantry left to match them with anyway.
Combining some fairly large bid, friendly allied pieces from ME, flown-in fighters, and modifications to the game to keep Axis income, can-opening and SBing to a modest limit, Moscow can be kept alive. If Moscow is still alive before Japan can sneak in the back door–the Allies may be able to invade Scandinavia, Leningrad, or come up in force from the middle east and stymy Germany further. Still, once Germany has all the Russian factories to work with, and most of Russia’s money, it is only a matter of time.
I would also caution that while there is much you can do in Yunnan, and with the 20 pieces in the east, that these plans assure Moscow’s fall, or at least anticipate it (knowing Moscow will fall you do something else).
Also, if you are ignoring Italy with UK and are using the USSR Cheezbombing strategy (taking somalia and tobruk with USSR) you’ll need some mechs and armor for that. I’ve seen this save the bulk of Russian income–but some players HR this out because…its cheezebombing.
Buy now; 1 fighter, infantry
buy later; artillery to match
if bids or mods give Russia a survival edge
buy now; 1 fighter, 1 tac
buy later; whatever you like