Flush,
A complex yet simple answer to your questions is, “I buy what is necessary to do the job required.” I know that sounds stupid but we can break down the role of Russia versus what you would like Russia to be.
The first move for Russia should be to put Germany on the defensive by removing key pieces from play (West Russia and/or perhaps a second territory of choice). Germany then has to choose how to deal with Russia. If the German player decides to do a Naval build, then I might continue to press the attack on Germany with Russia due to lack of funds on ground units. If Germany spends it money soley upon land units, then a more defensive posture in necessary.
Keep in mind that units are purchased to serve a purpose. These units may or may not give you the initiative to control the tempo and pace of the game. An unwise build may cost your team the initiative in the game (for example, if I see too much armor being built, I can turn the direction of the game). How much armor constructed will cost the game? Good question, that is where the wisdom and experience come into play.
I deal with Japan in a number of ways with Russia, but it is also in conjunction with the Western Allies. Germany can do a few things to disrupt the Allied naval movements, but you must exploit that with Russia to even the odds. Find out how and where the enemy is weak and exploit it.
As for artillery, I don’t see Russia needing too much of it. Russia needs a bit of flexibility to deal with Germany and Japan for the spearhead could come in multiple places (sometimes even at once). Also, Russia does not stand alone. Use your helpful Allies to deal with the unfriendly invaders on approach to Russia.
Russia should keep pressure upon Germany to not allow Germany to do a build up without losing all of its Eastern European territories.
Hint: you don’t do this with Armor.