If I’m going to spend IPCs on an IC I would wait until I have Western Ukraine (it should fall quickly), that way any units I build in W Ukraine and Ukraine could be with-in striking distance of Moscow. You don’t need more units to push into Russia, but you need more units to finish them off. That way if the allies fly a bunch of ftrs into Moscow you can build more units at the front, Romania isn’t going to help you in the later stages of your Moscow campaign. The only possible help a Romanian IC will give you later is if the allies try to come up though the Balkans mid game.
When I did it, i ofcourse filled all of the w ukraine IC as soon as i got it as well. I also built a minor in rostow and once I had Rostow, ukraine and stalingrad, I produced 9 infs there every turn, keeping the russian bottled up in moscow, while using the infs/arts to release my more mobile elements to attack the middle east from the north. Persia and iraq should fall once you come crashing down wtih 40+ units. The brit could perhaps challenge this. if UK has the minor IC in iran, iraq and in egypt from UK2/3, they should be able to have a lot of units being able to trade cauc and nw persia for a few turns.