New to WC4, some Questions

First of all, welcome to the forums! Now, to answer your question.

1. You can't get Guderian as your first general... at least not in a short time. The beggining path is (Messe)-Leeb-Konev/Guderian. You will get a lot of medals from achievements, domination battles, and also future campaigns. Hard WW2 Europe reward is 60 medals, WW2 Pacific is 45 on Normal and 75 on Hard.

2. You will essentially want to build every wonder that produces resources and increases number/rewards of task/invasions as soon as possible. The ones that reduce the cost of tech upgrades by 10% will also save you a lot in the long run, and I chose not to upgrade a tech tree until I get it. Those that affect trade as useless(at least until you get cap on a resource, since the deals are terrible), as are the ones which reduce task time.(for which you can you use clock trick)

3. There are only 5 generals (4 gold, 1 silver) for which you can get discounts, each one being specialized to command a different troop. To unlock them, you must first finish a special mission in their city, using your legion units. After that, you can get -20% and -40% discounts on them by completing another 2, more challenging missions. The 5 are: Rundstedt-Infantry commander in Vienna, Konev-Artillery commander in Moscow, Rommel-Tank commander in Berlin, Nimitz-Navy/Air commander in New York and Dowding-Air commander in London. Rundstedt, Downding and Konev can be got at somewhat easily at -40% if you have at least HQ level 20-25, while the other two require good tech and strong generals to be completed. All are very good for their price, but the best deals are for Konev, Rommel and Nimitz.

4. I'd say low health and defense is a bigger problem that low mobility. Firstly, you can get mobility upgrades for artillery pretty cheap and early in the game, which will make it move 2 hexes on flat, and that's ok. The 2 tile range, the firepower perk which prevents counter-attack and the raw damage are the main strenghts of howitzers. I ussually buy them when I don't have enough industry for building tanks, but have lots of money.