Re:German invasion of Russia in '39 scenario
1. Invade at the earliest possible chance. The force equation starts out in your favor and gets worse, you need to make as much hay while the sun is shining as possible. Hopefully you punish the Red Army badly enough that he cannot effectively take advantage of the freeze. You can also try to get some units out of the USSR proper the turn before, or get all the "freeze" victims (or most of them) in one plce so you can spend a single SA to replenish them back up to full.
2. The answer is as little as you can get away with.