As Germany in my first buy you should buy a maximum amount of infantry, because these will be needed on the Eastern Front and for defence of the West. You are going to need them. Later when you have decent defenses, you can switch to tanks, fighters or bombers to help out with attacks or defence.
I have a pretty standard opening with Germany. I try to destroy the Royal Navy as much as possible with aircraft and subs. I also attack the three Russian terroritories while keeping most of my armour on one spot, East-Poland. In that way you have a strong center and flexibility to attack in every direction.
I dont attack Egypt on round 1. First I bring up reinforcements to make sure my attack will succeed on round 2.
And perhaps later in the game I might consider buying a factory. It can be relatively safely placed in Poland, together with an AA. In that way infantry is quicker to the Russian front. But France is also an option, but you have to make sure the Allies dont capture it. It has to be very well guarded.
I usually dont really do much with weapons research. I find it a waste of resources. If you really have money to spare buy Bombers. They are very effective in killing the enemy with long range. A very helpful weapon for Germany considering the many fronts it has.
All in all, if Germany is pretty succesfull in the round 1 without too many losses, you are in a good spot to hold out, even if the Allies go full out on you. The German Airforce is powerful and will make any attack on your important territories a bloody affair for the Allies.