KV - Box turret, 107mm, hang behind the main group and close range sniper support is your game. You cannot turn fast enough if threats get up close to keep them at a distance. They are overpowered to pieces in Tier 5 max matches, it's hilarious.
Crew? Yes, 75% crew probably starts becoming affordable/desirable at Tier 5 and above. 50% crew is just desperate.
AP/HE always depends on the gun and your target. For example, the Heter's 10.5cm is clearly a gun meant to use HE (low AP penetration, high'ish HE damage).
A penetration type gun like your StuG III's Gun 7,5 cm StuK 42 L/70 which has 138mm AP penetration and 38mm HE penetration. It is clear that on most targets (up to Tier 5/6 Heavies and similar Mediums, maybe 7's too) that you should use AP. For targets above that, you're not likely to do much to them except bounce with AP so HE is a method which means you can deal some damage.
See '
When do I use Armour Piercing (AP) rounds and when do I use High Explosive (HE) rounds? " in my
FAQ. It needs updating but the AP/HE arguements are still relevant.