I'm looking to pick up my first Combat Mission game, and am torn between Final Blitzkrieg, Battle for Normandy, and Fortress Italy. Can anyone enlighten me as to the major differences between them other than what theatre they follow? I'm mainly curious about mechanics, the amount of units, and the amount of scenarios, alongside general enjoyment.
Alongside that, I'm also curious as to how exactly the engine upgrades work. If I buy, say, Fortress Italy, and want to upgrade to Upgrade 4, do I have to have purchased upgrade 3 prior to that, or can I just go right into Upgrade 4?