It has big worth to play the first three "introduction" scenarios too!
1., You collect extra prestige (scenario + surrenders).
2., You collect a lot of experience.
3., You collect kills, that means your core force get sooner heroes.
4., If you want (it is recommended), you can change your base core force to build a more usefull mix of units.
5., These 4 factors effects widely, what chances you have in the 99 turn Barbarossa scenario!
I must say, I look the game through the "filter" of Rommel difficulty. That means, prestige is allways scarce and I take all chance to manage it economical. If somebody plays it on lower level, maybe it is not as big importance, athrough victory can be achived in BE only, if you can manage it smart.
There can be possible SPOILERS, althrough I want to avoid them.
Poland
If you decided, that you will change your core force, you will have to fight slightly different. Honestly, it will be easier.

Your only enemy is time. So, if you want to take all non-important objectives, you have to hurry. OK, if you use the Soviet forces too, than it will no need for it, but I leave them there, as they are, because otherway it would be too easy. And I don't want to be in touch with the "commies" anyway.
Before starting the battle, you should plan ahead, what core force mix you want to have
after the Poland battle. The best core composition could be the following:
- No inf units overall (you will have enough in Barbarossa)
- 3 panzers: 2 upgraded, the Pz IIC to sell (too much prestige to loss if upgraded) and buy a Pz IV
- 2-4 arty, possibly 1 SiG of them
- 2-4 fighters
That means, from your initial core force, you will take over just the 2 Pz I's and the 2 fighters. All other will be sold (Also the 105mm arty, because it is better to sell it for prestige and buy a new 150mm one. Arty gains experience fast, so no need to worry). That means, while you have to pay attention to the other core units, not to be destroyed. The casualties, they sustain takes no prestige from your pocket. Just don't forget to give them green replacement between the scenarios to sell them for full price (keyboard shortcut for selling a unit: "D")
Beside of the timerun, the scenario is mostly easy and you can even replay it, if you doing not well enough.
Basic principles:
- Gain as much prestige with the above mentioned core units, as possible
- Try to avoid casualties (even more with your fighters) with the above mentioned core units
- Try to make as many units of the enemy to surrender, as possible. It brings extra prestige.
- Make a DV, it brings also extra prestige.